Previously, even non-admins had the option to override built-in
emojis in the `Settings Emoji` UI.
This commits essentially limits the functionality of overriding
custom and allows only realm administrators to
override built-in emojis with their custom emojis by adding an
authorization check in the backend.
It also adds relevant tests in `test_realm_emoji` which tests
for the cases where an admin and non admin tries to override
the built-in emoji.
Fixes#18860.
7dc56fdcf2