Allow subpages on group websites

[danielru]

Enable the use of subpages as it is implemented for events also on the group websites. See commit d754d21 for information.

[danielru]

Attempt so far in this branch

Problem: Entry from group.md file and Group's Main Page both appear, but should only be one.

Reason probably somewhere around this if statement.

[danielru]

Okay, thanks to @pancetta for pointing out how to fix this. Next problem, for some reason the subpage does not properly link to the main page and both appear as separate entities.

[pancetta]

1. You need to specify permalinks in the headers, i.e. permalink: /groups/leeds/ and permalink: /groups/leeds/research. Then linking between the subpages works.
2. To avoid separate items, you'll need to fix the index.html of the groups page, I guess. Look at the index.html from the events, there is a if-clause for that, see here. I leave that as a homework... sorry, just kidding!