Sometimes plugins need to have additional data or options available
when rendering custom markdown features/rules that are not available
on the default opts.discourse object. These additional options should
be namespaced to the plugin adding them.
```
Site.markdown_additional_options["chat"] = { limited_pretty_text_markdown_rules: [] }
```
These are passed down to markdown rules on opts.discourse.additionalOptions.
The main motivation for adding this is the chat plugin, which currently stores
chat_pretty_text_features and chat_pretty_text_markdown_rules on
the Site object via additions to the serializer, and the Site object is
not accessible to import via markdown rules (either through
Site.current() or through container.lookup). So, to have this working
for both front + backend code, we need to attach these additional options
from the Site object onto the markdown options object.
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.
