docs: what's new in v5.4

Explore: Fix JS error when switching between 2 prometheus datasources

- since the DataQuery rewrite, Explore starts by submitting its queries (Explore cant know if the queries are emtpty)
- The datasource intercepts an empty query and returns an empty list of time series, and also no table for the table query.
- The query hinter then received no series to analyse and was not guarding against this.
- This PR adds this guard.

Sort Prometheus range suggestions by length

Regression: fix handle of elasticsearch 6.0+ version

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…

- add sortText property to range completion items

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

when listing multiple gf-form elements this style is
applied to all elements except the first. By having the
margin on the right side there will always be some margin
between all gf-forms

Fix a typo in doc about alerting

Fix a typo

Fix invalid time range causes panic in Cloudwatch datasources 

Add ability to toggle visibility of graph series in explore section