Explore: Introduce DataQuery interface for query handling

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

updates time range options for alert queries

Fix errcheck issues

add some new options for the to value.
removes '1s' option for from since thats unreasonable low

closes #12134

Adds example timeline in docs for alerting for

- Queries in Explore have been string based
- This PR introduces the use of the DataQuery type to denote all queries handled in Explore
- Within Explore all handling of DataQueries is transparent
- Modifying DataQueries is left to the datasource
- Using `target` as variable names for DataQueries to be consistent with the rest of Grafana

Discovered implementation issues with the set utilities used in the
explore section after adding some tests. Added tests to reinforce the
expected behaviour of the utility functions.

Related: #13522

Co-Authored-By: bergquist <carl@grafana.com>

Co-Authored-By: bergquist <carl@grafana.com>

fixed issue with new legend not checking if panel.legend.show