DevOps Adoption change add groups to dropdown

What does this MR do?

This MR replaces the modal used to enable / disable devops adoption namespaces with a dropdown. This is a far more performant and scalable solution.

The dropdown displays a maximum of 20 records at a time i.e. the API request will fetch 20 groups based on the search term, and we'll first filter out already enabled groups from the list before displaying the remaining groups in the dropdown.

A 500ms buffer / delay has been implemented between typing and searching for results. This prevents searching on every keystroke.

Once a group is enabled, it will automatically be removed from the dropdown.

The instance level and group level feature have corresponding copy i.e. group on instance level and sub-group on group level.

Screenshots (strongly suggested)

Mobile	Desktop	With results	No results	Disabled	In action
					Screen_Recording_2021-06-14_at_11.53.04

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• 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.
• [-] This change is backwards compatible across updates, or this does not apply.

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 #329445 (closed) #333231 (closed)