Refactored group repository analytics

What does this MR do?

Related to #215135 (closed)

Refactored group repository analytics component to make room for adding more sections to the test coverage area. Moved the projects dropdown to its own component so it can be reused in an upcoming section.

Also moved the modal button to be inside of a card according to design.

Also updated the project dropdown to update the placeholder to show how many projects are currently selected, according to the design in #215135 (closed).

Screenshots

Before

After

Screen_Cast_2020-09-25_at_8.37.11_AM

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

• 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](https://docs.gitlab.com/ee/development/contributing/merge_request_workflow.html#definition-of-done