Bulk moderator operations on posts/comments

[Sjmarf]

Specifically, the ability to bulk remove posts and comments. Could be useful for removing large amounts of spam. This could be done in a number of ways:

• There could be a "Select" action in the ellipsis menu that lets you select which comments to remove. You should be able to both remove and ban the authors of selected posts/comments without having to reselect all of them again.
• The comment removal sheet could have a "remove all children" toggle (this isn't a problem for purging, which already removes all children).
• There could be a "remove all posts/comments created after..." option somewhere. This could be made more granular by adding a filter to only remove content from accounts created after a certain date.