Refactor Community Reviews (Observations) to use Tags

### Proposal

If one clicks to add a review...

A modified modal should open that shows some examples and allows the patron to search (and/or suggest new tags); something like:

The observations table should be modified to use a single value, which is the Tag key rather than the existing label and value pairs:
https://github.com/internetarchive/openlibrary/blob/master/openlibrary/core/schema.sql#L76-L77

The community reviews form should **also** open **after** the patrons adds a star rating, asking them if they want to add any tags to help other patrons evaluate whether the book is right for them.

### Justification

1. Community Review Tags aren't in the search engine (and are useful)
2. The "Observations" naming is confusing #10319
3. Community reviews are currently their own system (maintainability)
4. Community reviews form currently require multiple levels of categories and clicking, which should be collapsed into single tags (e.g. instead of hypothetical cases like "pace" -> "fast", it should appear as "Fast Paced" and use the Tag "community: fast paced"

### Breakdown

#### Requirements Checklist

* [ ] Switch observations db schema to
* [ ] Update observations UI to search for tags by name (drop "community:" from tag prefix)
* [ ] Open review form automatically after patron star rates book
* [ ] Add documentation re: #10319

#### Related files

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