Zulip converts :) to the 1F642 Unicode emoji and promotes the same emoji
in the popular section of the emoji picker.
Previously Zulip has labeled 1F642 as "slight smile". While that name
conforms to the Unicode standard (which describes the code point as
SLIGHTLY SMILING FACE), it didn't match our use case of the emoji.
If a user types :) or selects the first smile in the emoji picker they
probably mean to express a regular "smile" and not a "slight smile",
which raises the question why they are only smiling slightly.
This commit relabels 1F642 as 😄 and our previous 😄 263A as
:smiling_face:. Note that 263A looks different in our three supported
emoji sets, so it is not suited to be our "default smile".
This change does not require a migration since our emoji system stores
both unicode points and names and handles name changes transparently.