* (feat/explore): Support for new LogQL filtering syntax

Loki is adding syntax to support chained filtering.
This PR adapts Grafana to support this.

- Send only `query` parameter in loki request
- Automatically wrap search text in simple syntax, e.g., `{} foo` is
sent as `{} |~ "foo"`.


* Adapted to regexp parameter staying on in Loki

* Dont wrap single regexp in new filter syntax

* Fix datasource test

* Fallback regexp parameter for legacy queries

* Fix search highlighting

* Make highlighting work for filter chains

* Fix datasource test