Explore: Filter logs by log level

Remove confusing <> from variable intro

The existing intro makes it look like you should use `$<varname>` to refer to a variable. That of course is incorrect, and you should use `$varname`. In my experience using the <> marker around variable names should only be used when there isn't something else explicit to set it off. In this case we have `$`. Below, you can see the <> are also not used, because we have the `[[]]` to set it off. The inconsistency is confusing.

Related to #14182

chore: correct pause-all-alerts auth in docs

log the actual error if failing to update alert data

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

- add `onToggleSeries` to Explore Graph props
- toggling a Logging Graph series from the legend propagates its hidden series to Logs
- Logs translates hidden series alias to LogLevels
- Logs filters out hidden log levels

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