TRACKING: Emoji Picker popup improvements

[zee-bit]

We recently added support for reacting to messages with Emojis (other than the previously :thumbs_up:), ending at 19f1396. This was a combined effort of multiple contributors and multiple PR's (#913, #707). We decided to go with a simpler logic for now and add improvements later. Here's a list of UI/UX improvements that need to be worked upon:

• Highlight the reacted emoji in the list of original+aliases - e.g. for emoji +1, thumbs_up, like, if thumbs_up is the reacted emoji, then highlight it similar to how we do in footer autocomplete - +1, <thumbs_up>, like
• Allow reacting with aliases as well - Currently, we only allow reacting with original emojis (the first one in the list of original+aliases), unless the message was already reacted with an alias by others. We want users to be able to react with aliases of an emoji as well. e.g. for +1, thumbs_up, like, the user could further select any one from these to react with, not just +1.
• Update the EmojiPicker popup on reaction events: This should be fairly straightforward, because we already update the underlying data-structure (i.e.model.active_emoji_data) on reaction event. The only thing left is to update and render the EmojiPicker popup on reaction events optimally.
• Group emojis into different categories similar to the web app: The web app groups the emojis into categories namely (Popular, Smileys & Emoticons, People and body, etc), though this is not much of a high-priority right now, but there are users who would really want that sort of distinction in ZT too.
• Show the list of 'to-be-changed' emoji during the search: This point was raised in the closing comment of feature: Add Reactions to messages. #913, according to which it's not clear which emoji will be applied on closing the popup, during the search. We would likely want to have a fixed section of 'to-be-applied' emoji during the search. Please discuss further before working on this.