Show registration buttons only if adhocracy.allow_registration is set.

Registration buttons in header and overlay are shown only if
registration is allowed for the adhocracy installation
and an appropriate message is shown for each case.
Allow to add id attribute to watch buttons
Show tutorial step only if follow button is there (fixes #46)
Fix tutorial anchor (fixes #55)
Don't show norms in dashboard if norms aren't used anywhere (fixes #2)
Fix color and size of "argument is contra" (fixes #33)
Perform text is_deleted filter on database side
Fix proposal changes counter (fixes #306)

While at it, calculate count on database side.
Do not require valid email address in the feedback instance

First issue found with selenium tests :)
Remove outdated version fixations (Closes #307)
Allow installation timezone configuration (#44)

This allows setting the timezone as a configuration variable
`adhocracy.timezone`.

This variable is used in the helper functions `date_tag` and
`datetime_tag`. The latter was previously called `relative_time`.

This also adds a `<time>` html tag around pure date informations, i.e.
`h.format_date` has been replaced with `h.date_tag` all around.

This is the first part of issue #44. The dynamic javascript part remains
open.
Remove fixed height from register overlay.
Put proposal detail page sidebar in a box

This fixes #49 on the go.
Merge pull request #304 from wawe/hide-registration-buttons-if-disabled

Show registration buttons only if adhocracy.allow_registration is set.
Add allow registration placeholder to adhocracy.ini.in
Merge branch 'develop' of https://github.com/liqd/adhocracy into develop