Ugly: use non-standard instance urls to avoid same-origin-issue (fixes #621)

This makes the instance badge overlay use an iframe with the same url as
the outer instance list. This is possible as `/instance/xxx/member` urls
also work.

The proper solution would propably be to use CORS, but this isn't
trivial.

This fixes #621.
Fix title and breadcrumbs in instance badge template
Merge branch 'develop' into geo

Conflicts:
	src/adhocracy/templates/instance/badges.html
Remove unneeded body class attribute

This has been introduced in 67a2ade6123f3c4aa47bb1eb91b8ae04037f6963 in
the geo branch only, but has never been used and now appears in the
instance badge edit overlay by accident, so let's revert this.