Refactor curation queries

[bruvellu]

Updating curations for every taxon update is complex. The signals we set up are not handling well all the edge cases. Perhaps a simpler way is, instead of setting the curation relationship to every taxon, simply fetch curations from the ancestors. In this way, we don't need to handle the high burden of updating at every transaction.

This issue will attempt to solve the above and address #341. But before, there's a log of cleaning up to do. Mainly, moving the logic for taxon update, media synchronization, and curation update to Taxon methods. This is also a first step towards reducing the 6 redundant copies of the code in the views upload_media_step2, editing_media_details, editing_media_list, my_media_details, revision_media_details, my_curations_media_details.

[bruvellu]

The cleaning up is done with 3a4ecd2. This actually closes #341.

Now I need to find all the places where curations are being queried from Taxon and Media and update the logic to fetch all curations from the ancestors.

But I'll already merge the branch to develop to already push this code to staging for testing.