Move the rest EE files from app and lib

What does this MR do?

Move the rest EE files from app and lib. Namely:

• app/assets/javascripts/pages/projects/merge_conflicts/index.js
• app/validators/untrusted_regexp_validator.rb
• app/lib/banzai/filter/cross_project_issuable_information_filter.rb

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Frontend
  • Has been reviewed by Backend

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/3970 and https://gitlab.com/gitlab-org/gitlab-ee/issues/3971