dasboard_history: big refactoring of how the compare api looked, now a POST with a model for new & base version, refactored a simplified UI code as well, this was done to lay foundation to make it possible to do diff against current in browser unsaved version

dashboard_history: further css & markup fixes, removing style overrides & resuing existing styles and markup components

dashboard_history: SQL did not work when using MySQL, fixes to dashboard version numbering, so inserts start at 1, added migration to fix old dashboards with version 0

refactoring: trying to remove all the css overrides in history tab and make the styles more more inline with best practice css, and if needed use modifier overrides instead

refactoring: moved compare dashboard version command out of sqlstore, the code in this command did not use any sql operations and was more high level, could be moved out and use existing queries to get the versions

refactoring: moved dashboard history item formating (message) and fixed dashboard history migration issue, and removed from frontend tests that where no longer needed

operations instead of duplicating a bunch of get & save logic.

refactoring: Renamed dashboard version queries that wrongly had Command suffix, added missing OrgId to dashboard history commands and queries

* Update to latest oauth2 library using govendor

* Follow API changes

I added in a description since there was just TODO there originally.

chrome: an attempt to fix scroll issue with chrome, but this did not fix the problem as it seems to be caused by some interaction between graph rendering and scrolling, #8494

* remove X-Forwarded-* headers added by nginx when proxying data source & plugin requests

* properly handle X-Forwarded-For