Don't use general ValueError for specific wiki exception.

It masks bugs.
CSS Filter: remove unnecessary save_sr_image wrapper.
CSS Filter: move preview link/comment generation to r2.lib.pages.

These functions, while used in the CSS filter UI, are view code and
should be separate from the filter itself.
Disable subreddit stylesheets for old IEs.

Their CSS parsing is so poor that the risk of exploits is much higher.
CSS Filter: replace cssutils with tinycss2.

The aim of this patch is to allow a mostly equivalent subset of CSS
through as the cssutils-based parser did.

The subset of valid properties represented in this are the entirety of
CSS2.1 plus a handful of newer ones that are already in active use on
reddit.

Several IE hacks like the "filter" property, "_height", etc. are no
longer allowed.
CSS Filter: add security test suite.
Trigger a reflow on CSS preview so webkit browsers reset animations.
CSS Filter: unleash modern CSS.

Properties, functions, and at-rules from the following modules are added
to the safe lists:

* CSS Color Module Level 3
* Media Queries
* CSS Backgrounds and Borders Module Level 3
* CSS Values and Units Module Level 3
* CSS Flexible Box Layout Module
* CSS Text-decoration Level 3
* CSS Transitions
* CSS Animations
* CSS Transforms Level 1
* CSS Text Level 3

As documented on https://developer.mozilla.org/en-US/docs/Web/CSS/CSS3
Allow steps() function in stylesheets
Fix missed ValueError->WikiBadRevision translation.
CSS Filter: Bring back the size check.

Woops.
Remove obsolete css_validator_messages.