Users without "Browse Users" permission cannot edit project roles

(Copied from https://studio.atlassian.com/browse/JST-5368)

When editing project roles, if the current user does not have the "Browse Users" permission, the Frother-ised control doesn't know of any allowable values and does not allow arbitrary usernames to be entered. Everywhere else in JIRA, if the current user does not have the "Browse Users" permission, it displays a simple text field instead.

Video:
http://screencast.com/t/mNzyYPkqn4

This is a problem for Studio as the sysadmin user does not have this permission (sysadmin doesn't belong to the "developers" group - this is a separate issue and is tracked at https://studio.atlassian.com/browse/JST-5031).

This worked fine in 4.3.x and will be a pain point for Support if not fixed.

This bug needs work to make sure the multiuser picker gracefully degrades if the user of the component has no browse user permission. For example allowing submission of a list of comma separated usernames, or allowing usernames to be entered that are not part of the list (as there is no list) and failing out after checking with the server for that user.