refactor twitter service model

refactor conversations_controller a bit

avoid publishing empty posts

invalidate empty posts when created by a local user

add changelog entry

Fix regression in 404.html

Removing the conflicting green in colors

- This was a regression from pull request #4541
- Moved the red color to the section above to separate it from the rgb
  colors that aren't used

Moving colors in variables.scss to colors.scss

MrZyk pointed out that this information is preserved in git.

- Fixed the conflicting red color
- Currently using the darker red

Remove last post link

Conflicts:
	Changelog.md

Fix #4078 by refactoring 404.html

Add help link

Change .timeago color and align notification, fix #4502

Add a white background to images displayed in the lightbox

Conflicts:
	Changelog.md

and added changelog entry for bugfix #4005

Added jamisne tests for comments

Added 2 jasmine tests for fixing commenting

Bugfix: #4005 added changelog entry

I moved 404 to the assets to avoid loading font and images from joindiaspora.

 #- Can I move the design to a 404-style.css file which will be in the public repository too ?
 #- This pull request proposes to insert the two font files and the cat image, do you agree ? Depend of amazon seems really bad for me.

Improve tests2

Add mobile test: logged out user mobile