Correctly set active search facet

[keithbauer]

Description

Correctly set the active search facet

Motivation and Context

My previous hacky attempt didn't actually set the @opds:activeFacet in the OPDS feed. I thought the URL would pick up on it and do some background magic, but it didn't look like it did.

This now builds out a facet link by using cls.facet_link to ensure the activeFacet property is correctly set. But this generates a .../feed/... URL and we want a .../search... URL, so then we replace the href with what annotator.search_url builds before adding the facet to the OPDS feed.

How Has This Been Tested?

More manual testing and verifying the search OPDS feed for the active facet is set through +@opds:activeFacet: "true"

Checklist:

• I have updated the documentation accordingly.
• All new and existing tests passed.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (5c1b5d2) 92.56% compared to head (58bb111) 92.56%.
Report is 5 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1905      +/-   ##
===========================================
- Coverage    92.56%   92.56%   -0.01%     
===========================================
  Files          216      216              
  Lines        43659    43660       +1     
===========================================
  Hits         40412    40412              
- Misses        3247     3248       +1     

Files Changed	Coverage Δ
core/opds.py	94.93% <100.00%> (+<0.01%)	⬆️

... and 1 file with indirect coverage changes

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[mwbenowitz]

This does look like a cleaner implementation. I'm not familiar enough with this code to know if this is a permanent fix, but it looks good to deploy and confirm!