Group permissions (rebased onto develop)



This is the same as gh-2661 but rebased onto develop.

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This will add permissions changing warning and better handle errors on the group forms managed by owners and admins. To test it:
 - set up read-annotate (rwra--), import image and create annotations by another user. Then try to downgrade the permissions. It should show warning first

![permissionwarning](https://cloud.githubusercontent.com/assets/1065155/3339256/f894186e-f868-11e3-8b81-7f5f1c99abe9.jpg)

then when clicking save again it shouldn't allow you to change it

![notpossible](https://cloud.githubusercontent.com/assets/1065155/3339257/fb08bdde-f868-11e3-9ea2-b82134b2814a.jpg)

 - set up read-only group (rwr---), import image and create annotation as image owner. Then try to downgrade the permissions. It should allows you to do it.