Add setting to change default MR target project

What does this MR do?

1. Adds project_settings.mr_default_target_self column
2. Adds UI to allow user to set project_settings.mr_default_target_self
3. If project_settings.mr_default_target_self is true we use same project as default target for a new merge request. Otherwise we use old behavior where upstream project was used.

Related to #14522 (closed)

Database migration output:

Up

[2] pry(main)> require './db/migrate/20210331125111_add_default_target_project.rb'
=> true
[3] pry(main)> AddDefaultTargetProject.new.up
   (0.2ms)  BEGIN /*application:console,line:/lib/gitlab/database/with_lock_retries.rb:168:in `execute'*/
   (0.1ms)  SET LOCAL lock_timeout TO '100ms' /*application:console,line:/lib/gitlab/database/with_lock_retries.rb:168:in `execute'*/
-- add_column(:project_settings, :mr_default_target_self, :boolean, {:default=>false, :null=>false})
   (0.7ms)  ALTER TABLE "project_settings" ADD "mr_default_target_self" boolean DEFAULT FALSE NOT NULL /*application:console,line:/Users/dzaporozhets/.asdf/installs/ruby/2.7.2/lib/ruby/2.7.0/benchmark.rb:293:in `measure'*/
   -> 0.0018s
   (5.9ms)  COMMIT /*application:console,line:/lib/gitlab/database.rb:342:in `block in transaction'*/
   (0.2ms)  RESET idle_in_transaction_session_timeout; RESET lock_timeout /*application:console,line:/lib/gitlab/database/with_lock_retries.rb:168:in `execute'*/
=> true

Down

[1] pry(main)> require './db/migrate/20210331125111_add_default_target_project.rb'
=> true
[2] pry(main)> AddDefaultTargetProject.new.down
   (0.4ms)  BEGIN /*application:console,line:/lib/gitlab/database/with_lock_retries.rb:168:in `execute'*/
   (0.2ms)  SET LOCAL lock_timeout TO '100ms' /*application:console,line:/lib/gitlab/database/with_lock_retries.rb:168:in `execute'*/
-- remove_column(:project_settings, :mr_default_target_self)
   (0.9ms)  ALTER TABLE "project_settings" DROP COLUMN "mr_default_target_self" /*application:console,line:/Users/dzaporozhets/.asdf/installs/ruby/2.7.2/lib/ruby/2.7.0/benchmark.rb:293:in `measure'*/
   -> 0.0014s
   (0.5ms)  COMMIT /*application:console,line:/lib/gitlab/database.rb:342:in `block in transaction'*/
   (0.1ms)  RESET idle_in_transaction_session_timeout; RESET lock_timeout /*application:console,line:/lib/gitlab/database/with_lock_retries.rb:168:in `execute'*/
=> true

Screenshots (strongly suggested)

Project settings

When user choose "this project"

When user choose "upstream project"

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required) => !59118 (merged)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done