Minimal tag-based index

[matthijskooijman]

For rubrics at de stadsbron, a minimal version would be:

• [x] A fixed system page like the current index page
• [x] Which shows articles with one tag, which is specified through an URL parameter
• [x] Articles are displayed using a title and excerpt (needs #290), no image yet.
• [x] This page can then be linked from the tags displayed on any other page
• [x] This should be sorted by descending publication date, which is done by loading the page XML for each page displayed (this is inefficient, but removes the need for migrating these dates to the index initially).

Some code for this is probably already present in the https://github.com/PlanBCode/hypha/tree/datatype-tag_list branch (of which this is a slimmed down version).

Later improvements:

• [ ] Pagination
• [x] Include a featured image for articles
• [x] Include text pages and other datatypes
• [ ] Put a sort date in the index (copy of the publication date for articles) to allow sorting on that (requires running a migration for existing articles).
• [ ] Allow visitors to customize sorting behaviour.
• [ ] Create a new datatype to show such an index, to allow adding a header, footer, custom sorting, etc. This is #127.
• [x] More complex tag filter, might be needed to generate a front page. This might require some kind of "lang:taglabel" format, since not all tags might be available in all languages. This is essentially #310.
• [ ] Support hierarchical tags. This is #377.

[matthijskooijman]

For now, the plan is to maybe not extend the current tag page (much) further, but instead implement #87 to generate homepages and tag-based "dossier" pages. The code that generates the lists for #87 should probably be based on the current tag index, though, and could maybe be reused by the current tag index to generate the actual content.

Also, for now the idea is to just create these "dossier" pages separately, without linking the tags on a page to these pages (they would still go to the current tag index system page). On the longer term, we could maybe add (manual) per-tag configuration for this (e.g. "when you click this tag, go to (text)page").