Add moderation buttons in search popup

[talneoran]

Pull request checklist:

• CHANGELOG.md was updated, if applicable

Description

This PR adds moderation buttons to messages in a search popup opened from a split where moderation mode is active.

While this does seem to work for the cases I tested, I'm not really sure about the implementation and would like some input:

With the way the existing imlementation of adding the MessageElementFlag works, the ChannelView object "queries" the parent widget (by dynamic_cast), expecting it to be a split and checking for moderation mode in that split. The addition here is checking if the parent widget is actually a search popup, fetching the parent of the search widget which should be a split, and using that split to determine moderation mode. I'm just not sure if this is a good way of going about it. (A similar thing applies to determining the channel for performing the moderation actions when clicking the buttons)

To test, simply add whichever moderation buttons you would like to test, open a split with moderation mode enabled, open a search popup, and try the buttons.

Closes #2148

[pajlada]

LGTM 👍 Good quality of life change for moderators!