Hashtags extraction from objects. Background migration infrastructure.

Migrates objects.data->tag contents (embedded hashtags) to hashtags table (linked with objects via hashtags_objects table) via background migration which runs in parallel with the app and automatically enables improved hashtags timeline feature once complete (unless :pleroma, :instance, :improved_hashtag_timeline config setting is set to any value — false enforces legacy mode, true enforces improved mode even if migration is invomplete).

Until migration is not performed, hashtags timeline works in legacy mode (filtering by object-embedded hashtags) but always 100% correctly, without outages (automated tests are configured to verify hashtags filtering in both legacy and improved modes).

Closes: #2455 (closed) Closes: #1840 (closed)