Enables frozen string in the following:

* ee/lib/*.rb
* ee/lib/api/**/*.rb
* ee/lib/audit/**/*.rb
* ee/lib/banzai/**/*.rb
* ee/lib/ee/**/*.rb
* ee/lib/elasticsearch/**/*.rb
* ee/lib/generators/**/*.rb

Also enables frozen string in some vestigial
files of the ee/app directory.

Partially addresses gitlab-org/gitlab-ce/#47424.