Introduce message renderer

This new class replaces all existing server side message
rendering helpers and is the new global entry point for such
needs. All models with relevant fields now expose an instance
of MessageRenderer for those. MessageRenderer acts as
gateway between the existing processing solutions for markdown,
mentions and tags and provides a very flexible interface for
all output needs. This makes the API to obtain a message
in a certain format clear. As a result of centralizing the
processing a lot of duplication is eliminated. Centralizing
the message processing also makes it clear where to change
its behaviour, add new representations and what options
are already available.
use uuids for guid generation
Add warning to script/server if curl does not support AsyncDNS
drop rmagick, fixes #4594

This breaks the screenshot tool.

RMagick no longer compiles on recent ImageMagicks that have HDRI
support enabled.
Merge pull request #4900 from MrZYX/drop_rmagick

Drop rmagick
Fixed 4903 and not render vote form when user is not logged
Fix missing translation in #4905.
Merge pull request #4907 from goobertron/poll_i18n_fix

Fix missing translation in #4905
Merge pull request #4906 from hpetru/4903-poll-crash-spv

Fixed 4903 and not render vote form when user is not logged
Rewrite poll creation inputs
fixed federation issue due to missing delegation + test + some minor tweaks

fixed indenting

renamed test
Merge pull request #4911 from JannikStreek/feature/4904-poll_fed_issue

Fix: federation issue with polls
Merge pull request #4910 from hpetru/rewrite-poll-creator

Rewrite poll creation inputs
Reorder items on settings page and improve wording
Update cukes to be text-independent
Update changelog
Redirect to stream_path if is destkop
Merge pull request #4917 from hpetru/4806-new-status_messages-page

Redirect to stream_path if is destkop
Merge pull request #4883 from MrZYX/uuid

Use uuids for guid generation
Fix search field in the header of the mobile version
Remove mobile footer
Remove span12, main content now expand smoothly and the drawer become correctly displayed even in big resolution