Merge branch 'usergroups_page' into 'nightly'

Improve display of user's groups page.

* Show page notice as for subscriptions
* Show instructions when user has no groups
* Add explanation of remote groups and link to skilledtest wiki

Here is how it looks when a logged in user is not following groups yet

![login_nogroups](https://git.gnu.io/aroque/gnu-social/uploads/34b1bdcdb9e7277ec3deff4594061c59/login_nogroups.png)

After following a group, you still see the instruction notice. This way you have links to searching groups and creating a new one.

![login_groups](https://git.gnu.io/aroque/gnu-social/uploads/cb50689bcdfca7adc5cdcbe620487aa4/login_groups.png)

Finally when logged out:

![nologin_groups](https://git.gnu.io/aroque/gnu-social/uploads/40d81ab909321f8729c2696dd3938d46/nologin_groups.png)

The current version on nightly has basically no text at all.

See merge request !25