Re-introduce role-targeted broadcast messages

What does this MR do and why?

Reintroduce the changes in !77498 (merged) that was reverted in !80893 (merged) to mitigate Incident #6732.

This MR includes the original changes from !77498 (merged) as well as the required corrective actions outlined in #353186 (closed):

1. Ensure we guard the migration (using column_exists?) to not add the column in the MR twice: added in commit
2. Hide the new code in model layer behind FF: added in commit
3. Use present? instead of empty? when checking target_access_levels (it should be an array but was nil in stale cache data): added in commit
4. Add tests to cover cases when broadcast message comes from the cache: added in commit
5. Add regression test for the original issue: added in commit

New changes since revert

The following commits contain new changes that need review:

1. Add target_access_levels column to broadcast_messages conditionally
2. Do not return role-targeted messages when feature flag is disabled
3. Add tests for when broadcast messages comes from the cache
4. Ensure target_access_levels is an array before using it

Cache invalidation

Incident #6732 happened because:

1. Stale data: cached broadcast messages did not have the correct value for the newly added column target_access_levels (had nil instead of [])
2. NoMethodError: new code executed .empty? on the returned nil value for target_access_levels from the cache

1 is addressed in !81413 (merged) while 2 is addressed in this MR.

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