Update install guide link
Improved Plugins SCSS management

 - Moves the import of plugins for both mobile and desktop from common after discourse loading, allowing plugins to overwrite
 - Make desktop-option behave like the mobile-option: SCSS/CSS marked with that option will only be loaded for desktop from now on and ignored in mobile
 - Add variables-keyword, allowing plugins to ship and overwrite variables before they get imported by discourse (great for theming)
FEATURE: pre/code tags support in vanilla import
Also allow system_user to send pm's even if enable_private_messages is disabled
Merge pull request #2294 from techAPJ/security-link-update

Update install guide link in SECURITY.md
Merge pull request #2292 from ligthyear/sass-variables

Improved Plugins SCSS management
FIX: Don't show quote controls when the post id is missing.