Decide whether to deprecate Markdown in notifications

[Montoya]

Notifications are the only place where we still use markdown. If we drop it, we may want to allow a <Text> container with <Bold> <Italic> <Link> as allowed children.

Furthermore, the character count should be based on displayed characters (content of text ignoring JSX and URL param on Link).

[ajeetd]

Christian confirming who is using this, so that we can ask them stop/update and deprecate.