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.
A deactivated realm emoji should neither be accepted further as a
reaction nor its further occurences in a message be rendered as an
emoji. However, all the old occurences should continue to render
normally.
This commit removes all references to feature_flags.local_echo.
It's been a core feature for about four years, so I think we
can safely say the experiment was successful.:)
Harshit Bansal
Steve Howell