merge upstream/master
mod_admin: Prevent id confict between forms
doc: Add some documentation about startup environment variables
(cherry picked from commit 7b4a47719b7b90efbd2b804fdd5325a7831dab38)
mod_signup: Add fold notification for determining which fields to validate
mod_twitter: Change URL where one can resolve a Twitter ID
translation: Added more complete Russian translations

Replaced Russian "Yo" letters
doc: Explain API service naming in more detail

Thanks, Jeff Bell
doc: update frontend-growl cookbook entry.
core: added option to force the protocol of user facing urls.
This is useful when a proxy translates between https and http.
core: add support for 'optional include' templates.

Fixes issue #471.
mod_admin: optionally include country options.

Fixes #500.
core: add option 'use_absolute_url' to args and context of url generation (pages and media)
mod_seo: fix for showing the summary when there is no seo description for the page.
core: use z_context:abs_url/1 when redirecting to the main site url. Also make it a permanent redirect (301)
core: removed some ununsed var warnings.
mod_seo_sitemap: added category specific setting for update frequency and indexing priority.
mod_seo_sitemap: use 'all catinclude' to fetch the extra seo options.
Reverse the seo prio - highest prio on top.
Add extra 'is_visible' check to the sitemap listing to prevent empty items.
mod_seo_sitemap: generate language version links when mod_translation is enabled.
mod_seo_sitemap: fix for url generation, should page_url_abs.
mod_seo: let search engines only index pages with real translation.
This prevents double content on alternative urls, which happens when there are more languages enabled than translations made.
Only real translations should be indexed.