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.
Fixesgitlab-org/gitlab-ce#36066, fixesgitlab-org/gitlab-ce#31698
The only major difference with the EE version is the change from a light and dark logo to only a header logo
The dark logo wasn't used anyway, so it seemed to make sense to me to rename the field to the actual function of it
