Fix 'create new proposal' link for relative_url

Addendum to e74eaf6b0f0bda02dd3a69ac2e0065c6071c6503
Fix on-the-fly page modification for relative_urls

Addendum to e74eaf6b0f0bda02dd3a69ac2e0065c6071c6503
Use absolute URL in openid/xrds definition
Properly close non-void html tags

It seems like in html5, non-void html tags (those which may contain
content) need a proper closing tag, as the parser ignores the self-
closing space-slash at the end.
Remove redundant overlay-default div
Merge branch 'develop' of ssh://github.com/liqd/adhocracy into develop
Don't output microseconds in datetime attribute of <time> html tag
Added validator for proposal sort order.
Optimized validator code
Merge pull request #176 from yyazgan/develop

Added validator for proposal sort order
Merge branch 'develop' of ssh://github.com/liqd/adhocracy into develop
Add optional append_slash keyword argument to base_url function

Enabling append_slash makes the resulting URL end with a slash.
Fix comment editing and tag autocompletion with relative_urls

Addendum to e74eaf6b0f0bda02dd3a69ac2e0065c6071c6503
Fix comment editing after base_url change

Addendum to ad4c95357faeea8c96f28346fbde2f808162c290
Only revert to not-newest comment revision
typo
Allow deleting of newest text version

It is now also possible to delete the newest version of a text (page or
proposal description), if an older version exists.
Allow deleting of comment revisions

In addition to bed54e4a44c38e61f32eabdb32879f8751a64482, this commit
permits global admins to delete comment revisions from history.

This feature is restricted to global admins, as the revision objects are
deleted from the database itself, and not only marked as deleted.
Merge branch 'develop' into geo