Restructure layout of Add/Update approval rule form

What does this MR do?

This MR restructures the layout of the Add/Update approval rule form. This restructure improves the overall UX and also brings the UI more inline with our GitLab UI styles and standards.

Specifically it:

• Change inputs to be stacked rather than side-by-side
• Remove bolding from initial input description
• Changes the branches dropdown to GitLab UI's GlDropdown
• Fixes an existing console error
• Updates documentation images

Existing console error

Screenshots (strongly suggested)

Working without console errors

Screen_Recording_2021-03-09_at_09.35.36

Add form

Before	After

Update form

Before	After

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

1. Open a project and go to project settings, then open the Merge request approvals section
2. Check that adding and editing approval rules works as expected and looks correct
3. Go to an MR and edit it or create an MR
4. Add and edit approval rules under the Approval rules section
5. Add a license check to a project and check that works as expected
6. Add a security check to a project and check that works as expected

• 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 #301172 (closed)