There is a big difference between regular watched words and regular
expressions and this has been confusing in the past. This notice adds
an explanation.
This commit also reorganizes the code of the test modal.
It was not clear that replace watched words can be used to replace text
with URLs. This introduces a new watched word type that makes it easier
to understand.
This commit includes other various improvements to watched words.
auto_silence_first_post_regex site setting was removed because it overlapped
with 'require approval' watched words.
