Make changes queryable by annotation fields

[Mr0grog]

Similar to what we do with source_metadata in versions_controller, we should make the current_annotation field queryable in changes_controller. We should probably also expose a /api/v0/changes route (so you don’t need a particular page to search on).

This is especially useful to start thinking about now since #233 lets us pull in Analyst annotations from Google sheets and this querying would let analysts do a large-scale analytical look at their annotations.

[stale]

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[Mr0grog]

This is definitely still worth doing.

[Mr0grog]

Split out creating the basic route to #519. This can stay focused on what things should be queryable.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in seven days if no further activity occurs. If it should not be closed, please comment! Thank you for your contributions.