Add creator to custom emoji

What does this MR do?

We'd like to know who added a custom emoji, because we probably will allow the creator to delete/modify the custom emoji they added. So add this info to CustomEmoji.

Closes #284641 (closed)

Database

Migration output

#up

== 20210205134213 AddCreatorIdToCustomEmoji: migrating ========================
-- execute("DELETE FROM custom_emoji")
   -> 0.0028s
-- add_reference(:custom_emoji, :creator, {:index=>true, :null=>false, :foreign_key=>false})
   -> 0.0160s
== 20210205134213 AddCreatorIdToCustomEmoji: migrated (0.0190s) ===============

== 20210219100137 AddCreatorForeignKeyToCustomEmoji: migrating ================
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:custom_emoji)
   -> 0.0030s
-- execute("ALTER TABLE custom_emoji\nADD CONSTRAINT fk_custom_emoji_creator_id\nFOREIGN KEY (creator_id)\nREFERENCES users (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0024s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- execute("ALTER TABLE custom_emoji VALIDATE CONSTRAINT fk_custom_emoji_creator_id;")
   -> 0.0035s
-- execute("RESET ALL")
   -> 0.0006s
== 20210219100137 AddCreatorForeignKeyToCustomEmoji: migrated (0.0189s) =======

#down

== 20210219100137 AddCreatorForeignKeyToCustomEmoji: reverting ================
-- remove_foreign_key(:custom_emoji, {:name=>"fk_custom_emoji_creator_id"})
   -> 0.0048s
== 20210219100137 AddCreatorForeignKeyToCustomEmoji: reverted (0.0097s) =======

== 20210205134213 AddCreatorIdToCustomEmoji: reverting ========================
-- remove_reference(:custom_emoji, :creator)
   -> 0.0073s
== 20210205134213 AddCreatorIdToCustomEmoji: reverted (0.0074s) ===============

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team