Add display name for groups

[le-fractal]

Group ids are being used and shown throughout the app, and cannot be changed.
Having a display name that can be changed (by admins?), and that are displayed when the group name must be visible to the user would be great.

[le-fractal]

The issue was discussed in this owncloud bug :
owncloud/core#7918

[MorrisJobke]

Fix is in #8255