Now you can create a new file and select a target_branch != source_branch.
If the file that you want to create already exists on the target branch an error message is shown
A glDropdown is used to select and create a new branch instead of a text field.

Fix missing access checks on issue lookup using IssuableFinder

Split from !2024 to partially solve https://gitlab.com/gitlab-org/gitlab-ce/issues/23867

:warning: - Potentially untested
:bomb: - No test coverage
:traffic_light: - Test coverage of some sort exists (a test failed when error raised)
:vertical_traffic_light: - Test coverage of return value (a test failed when nil used)
:white_check_mark: - Permissions check tested

- [x] :white_check_mark: app/controllers/projects/branches_controller.rb:39
  - `before_action :authorize_push_code!` helpes limit/prevent exploitation. Always checks for reporter access so fine with
    confidential issues, issues only visible to team, etc.
- [x] :traffic_light: app/models/cycle_analytics/summary.rb:9 [`.count`]
- [x] :white_check_mark: app/controllers/projects/todos_controller.rb:19

- [x] Potential double render in app/controllers/projects/todos_controller.rb

- https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2024/diffs#cedccb227af9bfdf88802767cb58d43c2b977439_24_24

See merge request !2030

It adds a button to the branches page that the user can use to delete
all the branches that are already merged. This can be used to clean up
all the branches that were forgotten to delete while merging MRs.

Fixes #21076.