[Course Search] Update search index when tags are changed

[bradenmacdonald]

"As a course author, I want to be able to search for course content using tags."

Builds on #195 and #196.

Requirements:

1. When a user edits the tags of a course component in Studio (including a section, subsection, unit, problem, etc.), the corresponding document in the search index should be updated as well.
2. If the "reindex all courses/libraries" command is currently running, make sure it doesn't undo these changes. (The changes should either (A) be applied to the new index [perhaps in addition to the old/current index] or (B) be queued until after the reindex is complete.)

Ideally:

3. If the search engine (e.g. Meilisearch) is offline/down at the moment the change is made, the update will be queued; once the search engine is back online, the index will be updated.
4. This would work for when tags are changed on (v2) library content items too.

Notes:

• This Meilisearch API allows patching documents, so it can be used to update just the tags field without affecting the rest.