FIX: Replace use of auto_silence_first_post_regex with watched words (PR #12585)

This is another attempt at moving auto_silence_first_post_regex to watched words page. We tried to do this before in another tab, but the user experience was not great. This tries to use existing logic and add a ‘first_post_only’ flag to watched words.

For now, this flag can be used only for ‘require approval’ watched words.


Since this setting is a regexp, shouldn’t we also enable the watched words regular expressions as well?

We might want to do it only if there was a value in the first place though.