This mistake seems to have always been there, but it only resulted in
errors on the `/explore*.json` since they were the one that _actually_
relied on the local variables.

This removes remaining N+1 queries

Enables frozen string for the following:

* app/controllers/dashboard/**/*.rb
* app/controllers/explore/**/*.rb
* app/controllers/google_api/**/*.rb
* app/controllers/groups/**/*.rb
* app/controllers/import/**/*.rb
* app/controllers/instance_statistics/**/*.rb
* app/controllers/ldap/**/*.rb
* app/controllers/oauth/**/*.rb
* app/controllers/profiles/**/*.rb

Partially addresses #47424.

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

This changes the pagination of the "Explore" pages so they use a simpler
pagination system that only shows "Prev" and "Next" buttons. This
removes the need for getting the total number of rows to display, a
process that can easily take up to 2 seconds when browsing through a
large list of projects.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/27390

Extended ProjectFinder in order to handle the following options:
 - current_user - which user use
 - project_ids_relation: int[] - project ids to use
 - params:
   -  trending: boolean
   -  non_public: boolean
   -  starred: boolean
   -  sort: string
   -  visibility_level: int
   -  tags: string[]
   -  personal: boolean
   -  search: string
   -  non_archived: boolean

GroupProjectsFinder now inherits from ProjectsFinder.
Changed the code in order to use the new available options.