admin-ui: Render subnav responsive

Fix apyrgio/synnefo#280
admin-ui: Refactor scss files

There are two main  css files:
* 'main.css' for the dark theme
* 'main-light.css' for the light theme

Each corresponding scss file includes the files:

* 'settings': Common for all. Holds mixins and globally used
dimension settings and color palettes for both themes.
* 'bootstrap/variables': Used for dark theme.
* 'theme-light': Used for the light theme.
* 'global': Common for all. Holds all the project specific styles
* 'bootstrap.scss': Common for all. Contains all the bootstrap
inherited styles

Note: _theme-light.scss is a copy of bootstrap/_variables.scss with
different values for color settings.
admin-ui: Fix theme cookie path
admin-ui: Remove unused bootstrap components
Merge branch 'feature-admin-theme' into feature-admin