Fix suggested approvers on MR update

This was three issues combining:

1. https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/470 didn't apply the same coercion of the approvals_before_merge param to editing an existing MR as there was for creating a new MR. This meant that creating a new MR with the same number of approvers as the target project, editing it, not changing anything, and clicking Save would produce a validation error.
2. set_suggested_approvers wasn't called in a couple of places where we render the MR edit form (like the above).
3. The number of approvals used for clamping the approvals_before_merge param was based on the source project, not the target project (unlike the model validation).

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/18946, closes https://gitlab.com/gitlab-org/gitlab-ee/issues/710, and closes https://gitlab.com/gitlab-org/gitlab-ee/issues/711.