Dashboard: When saving a dashboard and another user has made changes inbetween, the user is promted with a warning if he really wants to overwrite the other's changes, Closes #718

Added basic auth to data source edit/create, add support for basic auth in data source proxy code, Closes #1510

Added different time period override to singlestat panel, added option to hide panel time override info, Closes #171

Worked on relative time and timeshift overrides for singlestat, still an issue for how to visualize the time override

Conflicts:
	src/css/less/sidemenu.less

More work on restoring features after moving to plugin model for datasources, no annotations work again #1276 #1472

A lot of work on backend plugin model for frontend components, right now for data sources, will enable dropin plugins for data sources and panels, #1472

further cleanup of styles to make sidebar more readable for light theme. This tweaked link colors, dropdown colors and table tweaks for things like Add API to make them feel a little more organized through shades of gray.