[meta] API: Grapify all the things!

Extracted from !6330 (closed).

The goal is port every API endpoints to the Grape DSL, so we soon can start generate docs.

Important note

We must not modify current API behavior (i.e. the tests shouldn't need any change).

Todo

• access_requests.rb: !6269 (merged)
• award_emoji.rb: !6629 (merged)
• boards.rb: !6876 (merged)
• branches.rb: !6630 (merged)
• broadcast_messages.rb
• builds.rb: !6877 (merged)
• commit_statuses.rb: !6879 (merged)
• commits.rb!6914 (merged)
• deploy_keys.rb
• deployments.rb
• environments.rb
• files.rb https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7806
• [ ] geo.rb (EE) No doc since it's internal. Note: This shouldn't appear in the generate doc
• groups.rb !7359 (merged)
• [ ] internal.rb => No doc since it's internal. Note: This shouldn't appear in the generate doc
• issues.rb: !7968 (merged)
• keys.rb: !6631 (merged)
• labels.rb: !7070 (merged)
• ldap_group_links.rb (EE) https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1002
• ldap.rb (EE) https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1003
• license.rb (EE) https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1004
• lint.rb
• members.rb: !6269 (merged)
• merge_request_diffs.rb
• merge_requests.rb !7358 (merged)
• milestones.rb !7373 (merged)
• namespaces.rb: !6632 (merged)
• notes.rb !7375 (merged)
• notification_settings.rb
• pipelines.rb
• project_git_hook.rb (EE) https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1006
• project_hooks.rb: !6963 (merged)
• project_push_rule.rb (EE) https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1005
• project_snippets.rb !7621 (merged)
• projects.rb !7456 (merged)
• repositories.rb: !7534 (merged)
• runners.rb !7377 (merged)
• services.rb!7970 (merged)
• session.rb !7381 (merged)
• settings.rb !8336 (merged)
• sidekiq_metrics.rb !7699 (merged)
• subscriptions.rb !7380 (merged)
• system_hooks.rb: !6861 (merged)
• tags.rb: !6860 (merged)
• templates.rb: !5717 (merged)
• todos.rb: !6875 (merged)
• tokens.rb !7378 (merged)
• triggers.rb !7378 (merged)
• users.rb: !7147 (merged)
• variables.rb: !6838 (merged)
• lib/ci/api/builds.rb

/cc @zj @connorshea @rspeicher