Modify options in the feast dropdown to include the feast and the range of folios on which the feast appears, rather than a single option for each feast-folio pair.

Remove duplicate tests of feast selector options on various views. Add single test for feast dropdown options.

Refs: 1707

…request

Fix behaviour of feast and folio dropdown selectors on Chant Edit page to show chants on a specific folio or for a specific feast so that the selector remains on the selected option. Fix blank headers for filtered lists of chants on Chant Edit page.

Modify instructions on the Chant Edit page when no chant is selected (these has previously suggested that only full texts and volpiano was editable via that page; this has been corrected to note that all chant fields are editable).

Modify SourceEditChantsView `get_object` method so that the edited chant is always set by pk rather than recency of creation. Previously, this had been effectively true when changes were made through the form, as Django parsed the URL parameter for chant pk into the request.GET queryset; however, since URL parameters cannot currently be set with the Django test client, tests of this views were relying on the recency of chant feature. Now tests and the live form rely on the same logic (the presence of a `pk` option in the form data itself).

Reduce extraneous querysets in SourceEditChantsView.

Refs: DDMAL#1707, DDMAL#1708

…ueries