Follow-up from "Backport of Multiple Assignees feature"

The following discussions from !11089 (merged) should be addressed:

@smcgivern started a discussion:

We should split this and assignee_id by issue / MR, so that we don't accept assignee_ids for MRs.
@smcgivern started a discussion:

case selected_users.length
@smcgivern started a discussion:

Same as above, we should only permit by issue / MR.
@smcgivern started a discussion:

This should probably say 'removed assignee' in CE, but it's really not very important.

- issue.assignees.take(max).each do |assignee|
  # ...

@smcgivern started a discussion:

We should probably move these blocks to their own _sidebar/_assignee partial
@smcgivern started a discussion:

This check if it's an MR is redundant now 😉
@smcgivern started a discussion:

We could do these in a single hash literal, then merge with options[:data].
@smcgivern started a discussion: (+1 comment)

Is this an array or an AR relation here? If it's a relation, we will perform an extra query for no reason. Try issue.assignees.to_a.any?.
@smcgivern started a discussion:

This should also be split into two partials.
@smcgivern started a discussion:

We should make this clearer for CE, because in CE there can only be zero or one assignee.
@smcgivern started a discussion:

a user -> the users?
@smcgivern started a discussion:

We could also use params.tap here?
@smcgivern started a discussion: (+1 comment)

Nice!
@smcgivern started a discussion:

user -> user