Reporter not allowed changing Reporter when limit_reporters = ON

When reporters are limited to their own issues and allowed to update
issues, they are able to change the issue's reporter (and gain
visibility on existence of other reporters).

This commit fixes the problem by only displaying the current reporter's
name (i.e without the [edit] ajax) when $g_limit_reporters = ON and the
current user's access level is greater than REPORTER.

Fixes #15770
Display blank instead of 'user0' in Assigned to field

When a user lacking the 'update_bug_assign_threshold' privilege (e.g.
updater) tries to edit an issue that has no handler, the 'Assigned to'
field shows 'user0' instead of a blank field as expected.

Fixes #12480