* Remove the existing approach for detecting signatures that only
worked in source mode; remove autoSignWikitext()
* Use the same approach for auto-signing in source mode as we have
already used in visual
* In both modes, detect whether the user has already typed a signature
at the end of their comment in the modifier, and if so, don't add a
signature
* Add test cases for the detection
Bug: T255738
Change-Id: I791d3035cb1ffc33ce3966d4617a25d08700c35b
