Logging: fix query parsing for selectors with multiple labels

- simplify selector parsing
- added tests

Explore: Scan for older logs

fix elastic ng-inject issue in query editor

Sometimes log streams dont return any lines for the given range. Would be great to automate the search until some logs are found.

- Allow Explore to drive TimePicker via ref
- Show `Scan` link in Logs when there is no data
- Click on `Scan` sets Explore into scanning state
- While scanning, tell Timepicker to shift left
- TimePicker change triggers new queries with shifted time range
- Remember if query transaction was started via scan
- keep scanning until something was found
- Manual use of timepicker cancels scanning

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