Resolve "Remove EE-specific code from `POST /api/v4/projects/:id/members`"

What does this MR do?

1. This moves EE-specific code from ProjectTeam to a new EE::ProjectTeam module.
2. This removes an EE-specific check in the API. The check is already present in EE::ProjectTeam#add_user/EE::ProjectTeam#add_users so it's redundant.

Are there points in the code the reviewer needs to double check?

Since the 405 (Not Allowed) response is not currently documented, I decided to just drop the EE-specific API check. If we want to keep this specific check, I can re-add it, though.

Why was this MR needed?

To better separate EE code from CE code, and to remove EE-specific code where possible.

Does this MR meet the acceptance criteria?

• API support added
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #4135 (closed)