Remove concordance model and related code

[dchiller]

CAO Concordance information has historically been imported from Cantus Database but never used in Cantus Ultimus. Now that CAO concordances are no longer present in CantusDB (see DDMAL/CantusDB#431), we need to remove them from Cantus Ultimus. As discussed in #797, if this concordance information is ever desired in Cantus Ultimus, it would need to be re-added from Cantus Index.

This PR:

• removes the Concordance model and references to it in other models
• removes portions of commands related to the importing of concordance information
• updates tests and test data to account for these removals
• removes references to concordances in the solr schema and search frontend

Also, historically, migrations have not been committed to version control (see #562). Migrations should be committed to version control, so we take the opportunity with this model change to do so, removing migrations from .gitignore

Closes #797. Closes #562.

[jacobdgm]

at some point, it might be nice to have a definitive statement here, rather than a question :P

[dchiller]

yes, i would also like to know how we will solve this problem 😂😂

[jacobdgm]

Like, in practice, I know a chant can span multiple pages.

Here, could we say something along the lines of, "a Chant belongs to an image page. In physical manuscripts, the last chant on a page often continues onto the next page. Cantus Database associates each chant with the page on which it begins, and Cantus Ultimus follows this convention [or substitute whatever CU actually does]"? A statement like this would communicate some of the nuance of the situation and how CU currently handles it, as compared to the current statement which communicates something along the lines of "it's complicated and weird, and darned if I know what to do about it!".

[jacobdgm]

Everything here looks fine to me. I left a couple of comments, but nothing that needs to be addressed before merging.