Update post editing capabilities

Edits

what \ by whom	Author	Mods	QMs
chat post	any time	no	no
Story post	n/a	no	any time
poll variant	until close*	no	any time
suggestion	until close	no	any time
dice roll	until close	no	any time

The asterisk means that the editing must be also gated on a quest-wide feature flag that's controllable by the QM (add it to the quest header).

Deletions

• QMs can delete anything at any time.
• Mods can delete any chat posts at any time.
• Players can delete their own chat posts, but not subordinate posts (poll variants, suggestions, rolls)
• Deleted items are visible to all QMs, all mods, and the item's author. For now we won't show deleted posts to anyone, and in the future this may be amended for site admins.
• (?) QMs and mods Site admins can restore an item.

Support in other modules

This issue also requires you to add the requisite fields to the API capability message. Must not be merged before the frontend support is ready.

Implementation plan

• most of the editing table: !95 (merged)
• poll variant editing blocked by default, unlocked by a quest flag: !96 (merged)
• deletions part 1 (individual posts): !121 (merged)
• deletions part 2 (cascade for subordinates): !125 (merged)
• restorations: TBD