is_indexable derived state not consistently updated

[mlbrgl]

Core problem

see #2813

Proposed solution

Remove is_indexable derived state altogether from the DB, and compute it on demand, maybe with the help of a view.

Alternatives

Make is_indexable an actual property of a chart, that would be arbitrarily check on or off, regardless of tags. After a preliminary conversation with @spoonerf, this option is less preferred as it seems to add an extra choice and complexity for authors.

Context

see #2813