Fix anura-engine group permissions.

Some changes to group organization need to be made to the anura-engine group.

[x] Jetrel needs to be made an administrator.
[x] I need to be made an administrator.
[x] Vultraz needs to be added to the group. (Currently, he is in a personal sub-group.)
[x] The imgui repository needs to be made group-writable.

anura-engine / anura