Create status checks form and branch selection

Implementation stages

Description	MR
Setup the store to handle the form	!61697 (merged)
Create status checks form and branch selection	👈 You are here
Create new create/update modals to show the form	!61702 (merged)

What does this MR do?

With #327638 (closed) we are pivoting the status checks from being developed as part of the MR approval rules section, to its own dedicated section. This is because status checks don't "approve" an MR but rather give the MR author/merger an idea as to whether the MR passes any external processes. Putting it in a separate section helps keep the distinction that you don't need a status check to pass to merge an MR like you do approval rules.

This MR creates the status checks form and branch selection:

• Form to enter the name, URL and branch of the status check
• Branch selection component to use the REST API to find branches in the project
• Validation of form values

Screenshots (strongly suggested)

Grabación_de_pantalla_2021-05-13_a_las_14.39.46

View	Light mode	Dark mode	Errors
Create
Update
Branches API error

Setup & testing

This form isn't used anywhere yet 🙈. It will be usable in the next modal MR. The screenshots ☝ show how it will be rendered in the modals

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 this is behind the :ff_compliance_approval_gates feature flag.
• [-] 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 #327638 (closed)