* chore: improve group mentions color handling
* Avoid storing the `GroupMention--dark` and light classes on the database.
* Avoid recreating YIQ logic on the backend.
* Improve text color flexibility through CSS variables.
* Apply fixes from StyleCI
* chore: tweak tests
* chre: unused import
Signed-off-by: Sami Mazouz <sychocouldy@gmail.com>