Draft: Add branch mirror model

What does this MR do?

Adds a new model and database table for branch mirroring.

Related #285459

Migrations

> rake db:migrate:up VERSION=20210701120832                                                                                 
== 20210701120832 CreateProjectBranchMirrors: migrating =======================
-- create_table(:project_branch_mirrors, {})
-- quote_column_name(:from_branch)
   -> 0.0000s
-- quote_column_name(:to_branch)
   -> 0.0000s
   -> 0.0118s
-- quote_table_name("check_197ea5a284")
   -> 0.0000s
-- quote_table_name("check_e84fd9e1d1")
   -> 0.0000s
-- quote_table_name(:project_branch_mirrors)
   -> 0.0000s
-- execute("ALTER TABLE \"project_branch_mirrors\"\nADD CONSTRAINT \"check_197ea5a284\" CHECK (char_length(\"from_branch\") <= 4096),\nADD CONSTRAINT \"check_e84fd9e1d1\" CHECK (char_length(\"to_branch\") <= 4096)\n")
   -> 0.0006s
== 20210701120832 CreateProjectBranchMirrors: migrated (0.0158s) ==============

> rake db:migrate:down VERSION=20210701120832                                                                               
== 20210701120832 CreateProjectBranchMirrors: reverting =======================
-- drop_table(:project_branch_mirrors, {})
   -> 0.0014s
== 20210701120832 CreateProjectBranchMirrors: reverted (0.0054s) ==============                                                                                                                                     

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• 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.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team