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.
FIX: Reflected post links were being deleted when they shouldn't and
vice versa.
Quote entire Ubuntu install guide
Merge pull request #2296 from riking/reformat-ubuntu

Quote entire Ubuntu install guide
move security link here
minor copyedit
move custom fields into its own concern
Add CustomFields to Post, Category and Group
fix double save missing error by using copy not actual reference
Add Custom Fields on Topics
Add Support for Arrays to CustomFields