Previously, realm emojis can override default emojis in emoji-picker,
if the user sets an exisitng emoji name to his/her custom emoji.
For clear understanding--
If a user sets an realm emoji with name `smile`, this leads to the
newly realm emoji override the existing default emoji `smile`.
To address such behaviour, Added a warning modal which requires
the user confirmation before overriding the default emoji.
Fixes#16913.
aryanshridhar