Show sherpa admins in forening user-listing
Handle undecodeable base64 auth param in API
Don't assume that expected fields actually are POSTed
Link to local ad redirection if applicable
Rename destination url appropriately
Separate ad URL logic into advertisement_url tag
Add message on structure tab for central foreninger
Save the forening id in session, not the entire object

This will give us one extra DB query on each /sherpa/ request, and
avoids problems with cache invalidation and schema migrations.

Note also that the new convention is to save an active_forening
attribute on the request object and access *that*, not the session
directly.