Cache Appearance instances in Redis

This caches the result of Appearance.first in a similar fashion to how ApplicationSetting instances are cached. We also add some NOT NULL constraints to the table and correct the timestamp types.

Database Checklist

When adding migrations:

• Updated db/schema.rb
• Added a down method so the migration can be reverted
• Added the output of the migration(s) to the MR body
• Added the execution time of the migration(s) to the MR body
• Made sure the migration won't interfere with a running GitLab cluster, for example by disabling transactions for long running migrations

Migration Output

== 20170809142252 CleanupAppearancesSchema: migrating =========================
-- change_column_null(:appearances, :title, false)                             
   -> 0.0005s                                                                  
-- change_column_null(:appearances, :description, false)                       
   -> 0.0002s                                                                  
-- change_column_null(:appearances, :description_html, false)                  
   -> 0.0002s                                                                  
-- change_column_null(:appearances, :created_at, false)                        
   -> 0.0001s                                                                  
-- change_column_null(:appearances, :updated_at, false)                        
   -> 0.0002s
-- change_column(:appearances, :created_at, :datetime_with_timezone)
   -> 0.0298s
-- change_column(:appearances, :updated_at, :datetime_with_timezone)
   -> 0.0221s
== 20170809142252 CleanupAppearancesSchema: migrated (0.0534s) ================

General Checklist

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together