I18n review in preparation for adding extension to translatewiki.net

i18 file:
* Use single ' instead of " where possible in i18n file
* Add description message
* Add user right message
* In German section: Use UTF-8
* Add some FIXME's

FIXME's:
* Define default values for globals like $ctAllowedGroup
* Use 'communitytwitter-' as message key prefix to avoid conflicts with other Twitter extensions
* Add message documentation, see https://www.mediawiki.org/wiki/I18n#Message_documentation

Once this is done, I will add extension to translatewiki.net

Change-Id: I5e2e1350ab486814cb93fb9740abb4ad9aa7ddd2