Users with group permission users should also create groups via API, fixes #6417

##### What does this MR do?

This PR determines ability to create a group, based on the `can_create_group` property of an user. This allows non-admins which are allowed to create groups, to also create them via API.

##### Why was this MR needed?

Currently only admins were allowed to do that.

What are the relevant issue numbers / Feature requests?

Fixes #6417
Prefer single-quoted strings when you don't need string interpolation or special symbols.