PostController: Add filter whitelist.
Don’t allow non-whitelisted format methods in Gdn_Format::To().
Don’t check the format against the whitelist in the post controller, just Gdn_Format.
Rename FormatWhitelist to SanitizedFormats and protect it.

This property is actually a static list of formats that are appropriate for Gdn_Format::To(). It shouldn’t really be modified by plugins.
Fix bug when enabling mobile themes.
Merge pull request #2231 from vanilla/hotfix/mobile-theme-async

Fixes fatal error when enabling mobile themes.
Merge pull request #2123 from vanilla/hotfix/whitelist-inputformat

PostController: Add filter whitelist.
Remove unused variable.
Fix the smarty_modifier_asset_url() function.
Merge pull request #2166 from vanilla/feature/decouple-request-root

Decouple the static asset root from the web root.
Merge branch 'master' of github.com:vanilla/vanilla into stage