Allow collaboration on merge requests across forks by default

What does this MR do?

Closes #23308 (closed).

The Allow commits from members who can merge to the target branch checkbox is unchecked when the MR is created by default. This MR changes the default to enabled.

Screenshots

Before	After

Migrations

20201214000000

db:migrate:up:

# rake db:migrate:up VERSION=20201214000000
== 20201214000000 ChangeMrAllowMaintainerToPushDefault: migrating =============
-- change_column_default(:merge_requests, :allow_maintainer_to_push, {:from=>nil, :to=>true})
   -> 0.0059s
== 20201214000000 ChangeMrAllowMaintainerToPushDefault: migrated (0.0110s) ====

db:migrate:down:

# rake db:migrate:down VERSION=20201214000000
== 20201214000000 ChangeMrAllowMaintainerToPushDefault: reverting =============
-- change_column_default(:merge_requests, :allow_maintainer_to_push, {:from=>true, :to=>nil})
   -> 0.0071s
== 20201214000000 ChangeMrAllowMaintainerToPushDefault: reverted (0.0115s) ====

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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

Related to #23308 (closed)