Refactor add_member to use search_and_select

What does this MR do?

Page::Project::Settings::Members#add_member

would previously call Page::Component::UsersSelect.select_user which has a redundant implementation of the select2 component.

This change instead utilizes Page::Component::Select2.search_and_select which has a built-in waiting for the search to be complete before continuing.

Rel: #212185 (closed)

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