Unify status checks and approval rules protected branch selector component

What does this MR do?

Status checks and approval rules both make use of a protected branches selector component. Currently these are separate. This MR unifies them so they both make use of one shared component.

Screenshots (strongly suggested)

No visual changes

Status checks	MR approval rules

Video

Screen_Recording_2021-05-27_at_12.16.03

Does this MR meet the acceptance criteria?

Setup & testing

Status checks

1. Enable the feature flag :ff_compliance_approval_gates
2. Go to [HOST]/gitlab-org/gitlab-test/edit and expand the Merge requests section
3. Add status checks if none exist and test the branch selector
4. Update a status check and test the branch selector

MR approval rules

1. Go to [HOST]/gitlab-org/gitlab-test/edit and expand the Merge requests (MR) approvals section
2. Add a rule if none exist and test the branch selector
3. Update a rule and test the branch selector

Conformity

• [-] I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
  • There are no user-facing changes
• [-] 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.

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

Related to #331130 (closed)