Add "bulk term delete"

[jzohrab]

Many requests for this through Discord, etc.

From the Terms page, check a bunch of terms, and click "delete".

UX concerns:

• maybe add an "actions" expanding section, similar to the "filters", because there may be other actions (e.g. bulk set status, bulk set parent, bulk follow parent?, bulk export to anki?) -- optionally, have a dropdown to specify the action, but that gets annoying because some actions like set parent require an extra text box. Maybe the dropdown would unhide a form for that particular action, and the other forms would remain hidden.

Test cases:

• if parent, children are unlinked, and any following statuses are set to "no"
• confirmation message

[jzohrab]

Added in develop branch.