Explore: make query field suggestions more robust

- drop invalid history items
- make highlighter more robust by defaulting to empty string on text to highlight

Explore: switching to logging should keep prometheus labels in case o…

Logging's query importer drops all labels that are not common to both datasources.

- keep labels if label lookup of logging datasource came back empty
- easier to remove labels than having to add them in the first place

Margin when listing multiple gf-form 

docs: signout_redirect_url description in auth overview

Currently the footer is based on top margins which looks odd on big screen sizes.
Also, the footer shows up on the top of the page on pages take a moment to load (while ng-view is rendering).

- True sticky footer based on flexbox
- Ensure footer stays at bottom while ng-view div does not exist yet

Explore: Introduce DataQuery interface for query handling