This commit extends the options which can be passed to
`PrettyText.markdown` so that which Markdown-it rules and Discourse
Markdown plugins to be used when rendering a text can be customizable.
Currently, this extension is mainly used by plugins.
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.
