user-import additional-fields

This allows to provide additional csv columns in user_import
and use them for replacing {0}, {1}, ... in the invitation mail
don't allow headings in user generated content to be as big as our own
redirect startpage to instance
Allow to hide proposals navigation item
Merge pull request #586 from xi/feature-show-proposals-navigation

Allow to hide proposals navigation item
Merge pull request #583 from xi/feature-heading-size-in-UGC

don't allow headings in user generated content to be as big as our own
Merge pull request #585 from xi/feature-redirect-startpage-to-instance

redirect startpage to instance
Fix poll tiles if not in instance

This is a fixup of 3b97349dcefb652555aff99ee3cc7a3696af8680 and
0da4579e2e11c3d435d64c0e2473cb9298ebff68.
Fixup, in case there aren't additional fields
Merge pull request #582 from xi/feature-user-import-additional-fields

user-import additional-fields
Fix typo
fixup c83f765
Merge pull request #584 from xi/feature-htmlfill-overlay-format

use overlay format with htmlfill (fixes #580)
form overlays

Whan an overlays only contains a single form this allows us to

-   close the overlay on cancel
-   reload the whole page (without overlay) on submit
add style for loading submit button
replace overlay_target by overlay_type

needed by next commit
fix triggering form overlay from query params
use form overlay on sectionpage edit
cross browser fix
fixup form overlays