Adds generic "Location index" page template, other exhibit display changes

[matt-bernhardt]

Why are these changes being introduced:

• The Libraries are starting to place exhibits into locations which were not anticipated during the initial development of this theme (which supplies page templates that are hard-coded only to two locations: the Maihaugen Gallery or the Rotch Library). Rather than copy-pasting these templates to result in more bespoke code, this should be abstracted to something more sustainable.
• The way that location information is defined in custom fields is likewise tightly bound to these two locations, making the introduction of new locations a step that requires developer attention.
• Some Exhibits are also anticipated with no firm end date, but our current page templates do not support data of this type.
• All the templates which provide the current behavior have some technical debt that is flagged by our linter.

Relevant ticket(s):

https://mitlibraries.atlassian.net/browse/uxws-1396

How does this address that need:

This PR does several things in order:

1. Any file that is edited during this work is cleaned of legacy problems as flagged by PHPCS. This should result in our linter having no issues with the affected lines (or at least to make any issues more easily reviewable as exceptions).
2. Introduces a new Exhibits | Location Index page template, derived from the existing Maihaugen and Rotch templates. Instead of having a hard-coded location value, however, it loads the targeted location from the post's Category assignments.
3. The new template, as well as one legacy template which loads all exhibits across locations, are updated to anticipate that the end date for Exhibits is now optional.
4. A small bit of code cleanup for the "show all exhibits" template, which somehow wasn't flagged before step 3.
5. A new helper function is added to functions.php, allowing page templates to look up location information according to the new data model. The need for this helper function is driven by the fact that some exhibits will be placed in a catchall category, with the actual name of the location stored in a separate field. Additionally, some exhibits may be placed into multiple locations. Navigating all these conditionals seems like something for a helper function, rather than repeated logic across multiple page templates.

Document any side effects to this change:

• The helper function is a bit longer than our linter prefers.
• The helper function returning an array of values is a bit different from how WordPress normally operates, where get_the_* functions typically return strings rather than arrays. Given how related the return values are, however, and the need to have a single returned entity, I hope this is acceptable.
• The choice to use page templates for this feature, rather than leveraging category pages, is a judgement call. Hopefully future-us won't end up cursing this decision.
• A future PR will remove the legacy page templates entirely, but we can't make that change until the pages currently on those templates have been moved.

How can a reviewer manually see the effects of these changes?

This branch has been deployed to the staging server.

Todo:

• [ ] Documentation
• [ ] Stakeholder approval

Requires new or updated plugins, themes, or libraries?

YES - there's a coordinated PR for the Custom Child Post Types plugin, which has also been deployed. MITLibraries/Custom-Child-Theme-Post-Types#5

Requires change to deploy process?

NO

[matt-bernhardt]

(This thread of comments may get deleted prior to code review)

The first commit results in the following diff between the two bespoke page templates:

% diff page-exhibits-maihaugen-gallery.php page-exhibits-rotch-library.php                                       
3c3
<  * Template Name: Exhibits |  Maihaugen Gallery
---
>  * Template Name: Exhibits |  Rotch Library
47c47
<                           'value'   => 'Maihaugen Gallery',  // Value for comparison.
---
>                           'value'   => 'Rotch Library',  // Value for comparison.
63d62
< 
101c100
<                       'value'   => 'Maihaugen Gallery',  // Value for comparison.
---
>                       'value'   => 'Rotch Library',  // Value for comparison.
117d115
< 
155c153
<                       'value'   => 'Maihaugen Gallery',  // Value for comparison.
---
>                       'value'   => 'Rotch Library',  // Value for comparison.