The user mention regex was checking for multiple lines,
so it broke when the user mention was on a new line.
This changes the regex AND adds a couple tests to
test inline markdown regexes.
Unicode emojis when rendered should display canonical short name.
Similarly, the alt text should be of the format `:<short_name>:`.
For both of these we currently display the actual unicode symbol.
As some systems don't have the fonts necessary for displaying them
properly, they are rendered as empty square blocks. This commit also
ensures that the markup generated for emoji generated by canonical
name and by an unicode emoji is same.
Fixes: #5555.
