Bug 26738: (bug 24819 follow-up) Fix manager selection

Bug 26738: (bug 24819 follow-up) Fix manager selection

Bug 23590 added a new feature to select the manager of a suggestion.
One month later bug 24819 added the ability to pick the suggester.

This second patchset broke the manager selection.

This patch simplifies the way the suggester is selected, using the
generic way and mimicking what is done for the manager.

Test plan:
- create a new purchase suggestion from within acquisitions (suggestion.pl?op=add)
- click "select manager," search for user, click Select
- see that the user you just selected shows under "Created by,"
- see that "Managed by" still says "You"
- modify the suggester
- save your suggestion
=> Everything is saved correctly

QA will test the permission alert:
Edit suggestion.tt and remove "&permissions=suggestions.suggestions_manage"
Edit the suggestion, select a manager, pick a patron in the list who
does not have sufficient permissions, save
=> you get the alert

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>