Digest search functionality

[benlk]

Changes

This pull request makes the following changes:

• [x] custom template for the Digest category
• [x] search box
  • [x] search box
  • [x] filter buttons for region
  • [x] datepicker input
• [x] search filtering
  • [x] by words passed in the search box
  • [x] by the region-specific checkboxes
  • [x] by a date picker
• [x] also affecting LMP? Remove LMP, increase query size to 20.
• [x] styles

Form in Firefox:

Firefox's datepicker

Inputting information in Safari, which lacks datepicker support (https://bugs.webkit.org/show_bug.cgi?id=119175 and https://caniuse.com/#feat=input-datetime)

Why

For #77

Testing/Questions

Features that this PR affects:

• /category/digests/

Questions that need to be answered before merging:

• [x] Is this PR targeting the correct branch in this repository?
• [x] do the date filters work? I did some testing and it appeared to work
• [ ] do we worry about the paradoxical situation where the "before" date is before the "after" date?
• [ ] does this need a polyfill datepicker for Safari or other browsers that don't have a native date picker for input type="date"? Safari supports input validation, so we can use :invalid as a selector to reveal additional information, as this PR does.

Steps to test this PR:

1. check out the PR, using a recent DB from production
2. go to /category/digests/
3. perform various searches:
   • by word
   • by date
   • by region
   • by any combination thereof
4. Examine each search's results to determine whether the returned posts match the query

[joshdarby]

@benlk Where should results for the search be appearing? This is what I see after each search I've tried

[joshdarby]

@benlk mentioned that I needed to actually set each post in the appropriate taxonomy instead of post category, which ended up working.