Follow-up from "Namespace license checks for multiple assignees"

The following discussions from !1916 (merged) should be addressed:

• @DouweM started a discussion: (+3 comments)

  I think we can disable reassign outright if the issuable doesn't support multiple assignees, since in this case it's the same as assign. I don't think it's in CE either.

• @DouweM started a discussion: (+2 comments)

  Do we still need (CE) here?

• @DouweM started a discussion: (+1 comment)

  Since this spec is EE specific, can it go into a separate file?

• @DouweM started a discussion: (+1 comment)

  I think we should backport as much as possible in this file, and just know that in CE, allows_multiple_assignees? will always be false and point that out with some code comments.