Enable more frozen string in app/controllers/

What does this MR do?

Enable frozen string literals in:

• 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

Why was this MR needed?

Because frozen strings require fewer memory allocations, and memory efficiency is a good thing.

What are the relevant issue numbers?

Partially addresses #47424 (closed).

Suggested Reviewers/Approvers/Mergers

@stanhu @rymai

(@godfat is on vacation)