Automatically load rest of alphabetical index on vocabulary home page

[osma]

This PR fixes #1108: it wasn't possible to scroll down to see the full alphabetical index of the first letter (typically A) on the vocabulary front page. The normal mechanism that triggers an AJAX request when scrolling down to the bottom of the first 250 results wasn't triggered on the vocabulary home page; it only worked on the alphabetical index page.

As analyzed in this comment the reason is that the callback for loading more lines isn't properly activated. The if check is too strict, it only works on the index page. I avoided the if check altogether and simply targeted the sidebar-grey-alpha CSS class, which is specific to the alphabetical index listings, instead of the more generic sidebar-grey class.

[codecov]

Codecov Report

Merging #1134 (281e00d) into master (a201ace) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1134   +/-   ##
=========================================
  Coverage     67.90%   67.90%           
  Complexity     1583     1583           
=========================================
  Files            32       32           
  Lines          3889     3889           
=========================================
  Hits           2641     2641           
  Misses         1248     1248           

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[kinow]

Tested with the containers from #962 , found no issues navigating the interface, scrolling, etc. Everything seemed to work fine.

[osma]

Thanks for testing @kinow!