Allow generic instance setting overwrite

All settings can now be overwritten on an instance base by

1. activating allow_overwrite on a setting, e.g.

    adhocracy.comment_wording.allow_overwrite = true

2. using it, e.g.

    adhocracy.comment_wording.aalen = true

For some settings, which commonly use instance overwrites, this will be
enabled in adhocracy.config.DEFAULTS.
Use generic instance overwrite for adhocracy.hide_individual_votes
Allow instance overwrite 'adhocracy.listings.instance_proposal.sorting'
Allow instance overwrite 'adhocracy.comment_wording'
fixup 42d1ce67b7207414eca8dc5c58ab3807ba0afd04
Merge pull request #915 from nidico/feature-generic-instance-options

Generic instance setting overwrites
Add permission poll.show_result with per instance configution
Don't show proposal rate poll scores if not allowed
update moment.js to 2.7.0
i18n: add traditional chinese (taiwan) (by charles chuang)

While transifex and moment.js use the locale code zh_TW, babel claims
that zh_TW is not a real locale and zh_Hant_TW should be used instead
(search for "zh_TW" on http://babel.pocoo.org/docs/api/core/).

This is where all the soft links are coming from.
add zh_TW locale alias
fix whitespace issues

revert part of cbf57b1e
fixup whitespace issue fix
Merge pull request #916 from nidico/feature-hide-results+1

Hide proposal rate_poll results (experimental)
Allow to hide proposal comments
temporarily fix pluralization

we can switch back to upstream when
https://github.com/camptocamp/c2c.recipe.msgfmt/pull/3 has been merged.
Merge pull request #919 from nidico/feature-hide-comments+1

Allow to hide proposal comments
Merge pull request #918 from xi/feature-i18n-zh_TW

translation: traditional chinese (taiwan)
remove unneeded exception variable
Add data-baseurl to overlay body as well