Allow to show all open proposals in milestone (fixes #562)

Add a boolean attribute "show all open proposals" to milestones, which
makes all open (not frozen, see #559) proposals show in the proposal
list of future milestones. For milestones in the past, this doesn't have
any effect.

As this will only be rarely used, the feature is by default and has to
be enabled throgh the configuration option
`adhocracy.milestone.allow_show_all_proposals`.
bump migration version
Merge pull request #567 from nidico/feature-562-milestone-show-all-proposals

Allow to show all open proposals in milestone (fixes #562)
Allow using user ID in messages

hhucn/adhocracy.hhu_theme#431
add page options to model
create sectionpage template and css
hide subpages from index for sectionpages
recursively calculate latest activity time
check is_sectionpage recursively
create page comments view
hide pro/contral/neutral in comments form for norms
create amendment view
fix proposal create ret_url
apply Page.allow_selection option

I was not completely sure about the scope of this task.

If I have gone to far in some places it should be simple
to revert my changes
apply Page.allow_comment option

I was not completely sure about the scope of this task.

If I have gone too far in some places it should be simple
to revert my changes
Allow to render markdown text

I searched for a canonical way to render a text
but there seem to be dozens of ways to do that.
pep8
Fix socialshare text for page variants
Add optional query_string parameter to base_url function
Take query string into account in login_redirect_url