Redirect to the new default branch from a deleted default branch

What does this MR do?

Adds a column to store the old name of the default branch for a project; stores a value in it every time the default branch changes; and sets up a redirect from the old name to the new name if the old one doesn't exist in the repository any more.

For now, only trigger this for projects/blob#show and projects/tree#show. However, we may want to roll it out to a number of other controllers in the future: #329100 (comment 620834617)

When it all works together, we get pages like this when visiting, e.g., http://localhost:3000/gitlab-org/gitlab/-/blob/foo-master/README.md:

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Information for database reviewers

db:migrate

== 20210705124128 AddProjectSettingsPreviousDefaultBranch: migrating ==========
-- add_column(:project_settings, :previous_default_branch, :text)
   -> 0.0020s
== 20210705124128 AddProjectSettingsPreviousDefaultBranch: migrated (0.0020s) =
== 20210707173645 AddProjectSettingsPreviousDefaultBranchTextLimit: migrating =
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0003s
-- execute("ALTER TABLE project_settings\nADD CONSTRAINT check_3a03e7557a\nCHECK ( char_length(previous_default_branch) <= 4096 )\nNOT VALID;\n")
   -> 0.0025s
-- current_schema()
   -> 0.0002s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- execute("ALTER TABLE project_settings VALIDATE CONSTRAINT check_3a03e7557a;")
   -> 0.0020s
-- execute("RESET ALL")
   -> 0.0005s
== 20210707173645 AddProjectSettingsPreviousDefaultBranchTextLimit: migrated (0.0185s) 

db:rollback

== 20210707173645 AddProjectSettingsPreviousDefaultBranchTextLimit: reverting =
-- execute("ALTER TABLE project_settings\nDROP CONSTRAINT IF EXISTS check_3a03e7557a\n")
   -> 0.0019s
== 20210707173645 AddProjectSettingsPreviousDefaultBranchTextLimit: reverted (0.0084s) 
== 20210705124128 AddProjectSettingsPreviousDefaultBranch: reverting ==========
-- remove_column(:project_settings, :previous_default_branch, :text)
   -> 0.0052s
== 20210705124128 AddProjectSettingsPreviousDefaultBranch: reverted (0.0066s) =

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

To test this locally, take a random project in GDK and create a new branch that is a copy of master (I called mine foo-master).

Change to that as the default branch; previous_default_branch will then be master, but since master exists, no redirect will happen.

Change the default branch back to master. Still no redirect, but now previous_default_branch will be foo-master.

Delete foo-master. Now visiting a blob, tree, etc, URL will redirect you to the equivalent page for master.

Related to #329100 (closed)