Permissions dialog: simplify with dropdown to select users and groups

[enfJoao]

I'm submitting a ...
[x] feature request

...about
[x] Content Types or data management

Current behavior
Public forms require admin to set permissions for data access. Since all users can view, and only admins cam edit the page, this leaves content editors with only one choice: permissions by role or user.
The current setup requires the admin to manually query the database for users and roles ID and input the ints in a text fields, comma separated.

Expected behavior
2sxc has evolved so much since these beta feats where implemented, that jumping into a fully featured dropdown multiple selection seems a very small step.

[iJungleboy]

Here are the steps we would need to implement this done correctly:

1. Create two DataSources, one to provide user groups, one to to provide Users (I believe the second one exists)
2. Create a query which accesses both and merges the result
3. Change the input field to become a string-dropdown-from-query using that query

All in all I'm afraid it still almost 1 day of work done properly. So ATM not a priority, but we're open to contributions!

[iJungleboy]

Needs sponsoring, so added to #2107 and will close unless someone contributes or sponsors this.