Add search field for billable members table

What does this MR do?

Adds search by name for the billable members table.

This is the Front-End implementation MR for #262875 (closed).

How to test

In GitLab, go to a group that has billable members. Then Settings > Billing, and in the subscriptions table click See usage.

Screenshots

Before	After

Too few characters typed	No users found

Video

Screen_Recording_2020-12-23_at_23.55.41

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

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