Add or remove blocking merge requests

What does this MR do?

Allow blocking merge requests to be configured

GitLab has model-level support for the idea of one merge request "blocking" another from being merged - the second MR can only be merged once the first one has, itself, been merged. We also show any existing blocks in the merge request widget.

This MR adds support for adding and removing these blocks when creating or editing a merge request.

Frontend changes mostly by Sam Bigelow <sbigelow@gitlab.com>

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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

Part of #9688 (closed)