Fix "Browse Chants" page "Source" drop-down bug

[lucasmarchd01]

This PR addresses the bug that was found on the chant-list page.

When the user would select a different source from the drop-down menu, instead of being redirected to
source/<int:source_id>/chants/ they get sent to /source/<int:source_id>/chants/?folio=&feast=&search_text=&genre=&source=<int:new_source_id>

There was some discussion in #1336 about whether this functionality is useful to users on the site. Perhaps the best option is to remove it altogether, but in the meantime it should work as expected for users that might be browsing this way.

The code modified includes the chant_list.js file which handles the filters. The page will now redirect correctly and the source parameter is no longer used in the URL. Additionally, the current source that the user is browsing is correctly displayed as the selected option in the drop-down menu.

Example after selecting a new source from the drop-down menu:

Fixes #1336, Fixes #717

[jacobdgm]

I suggested one small tweak to a variable name, but on the whole, this looks very good. Thanks!