Let emoji dropdown menu support tabs and custom emoji

Problem

This issue makes it so custom emoji become discoverable and have a place in the emoji dropdown menu.

Scope

• Introduces tabs
• This issue includes a new sort order, as it introduces tabs (originally https://gitlab.com/gitlab-org/gitlab-ce/issues/21443)
• This issue may delete the functionality of not being able to upvote one's own creations if that makes things easier to implement/is easy to do: https://gitlab.com/gitlab-org/gitlab-ce/issues/43283
• Emoji previewer/enlarger similar to Slack
• Add in custom emoji tab

Design

Sections and icon usage:

Section	Icon
Frequently used	clock
People	users
Nature	nature
Food & drink	food
Activity	dumbbell
Travel & places	car
Objects	object
Symbols	heart
Flags	flag
Custom	tanuki

Note: Icons may be visually different from the mockups!

Default aware emoji menu:

• Includes handy reactions + hint display

When scrolling:

• Tabs change while scrolling
• Tabs update scrolling position

Bottom/custom section:

Search:

• Search bar is sticky positioned (doesnt scroll)
• Search bar does not interact with tabs, but will render a separate section with only the fuzzy search results visible
• Section header is Search results

Selections:

• Hint changes to chosen emoji + shows name and shortcut phrase(s) (can be multiple if we implement aliases)

Skintone menu/chooser:

• Button expands horizontally when clicked
• Hint changes to point to the menu/chooser + changes skin color