The tags prompt was added for the Fizzy Do command system, but became
dead code when commands were removed in 89af9066. The commands_prompt
was cleaned up but tags_prompt was missed.
Remove the Tag::Attachable concern, Prompts::TagsController, its views
and route, and the tags_prompt helper. Tag still includes
ActionText::Attachable directly, which is needed by the data transfer
system.
The board-scoped mention prompt was returning all users with board access,
including those marked as inactive. This adds the `.active` scope to match
the behavior of the account-wide mention prompt.
Fixes https://app.fizzy.do/5986089/cards/3861