CSS improvements for easy themeing

- more extensive usage of variables
Merge branch 'dev' into feature/themeing_improvements
Fixed position of top menu drop down
Merge branch 'dev' into feature/themeing_improvements
Easier theme handling

- more sass variables
- use new font (Lato) in the content area
Fixed position of add symbol in wp form for

target version and category
Fixed 3547: Change color of Apply button

in Activity
fixed default font color
3409: New Layout for fallback Login page
Added new version of the openproject icon font

- new icon for backlogs
- new arrows
Changed icons in projects menu

3667, 3546
new arrows for drop down in top menu
fixed cuke for new login form
Merge branch 'feature/themeing_improvements' of github.com:opf/openproject into feature/themeing_improvements
Merge branch 'dev' into feature/themeing_improvements

Conflicts:
	doc/CHANGELOG.md
Merge pull request #817 from opf/feature/themeing_improvements

themeing improvements, frontend changes