Fix template variables multi-select

[aangelisc]

Allow multi-select of template variables in the builder editor.

The problem here was that we were quoting the template variable values. I've tested this with multiple filters/group bys and the quoting doesn't seem necessary so I've removed it.

Let me know if you find any cases where this may not be the case!

Updated the docs to reflect that these variables will now work in the builder.

Also, a minor change to ensure there is a table selected in the query editor before we display the error asserting that there is no table schema.

Fixes #823

[github-actions]

Use the following command to run this PR with Docker at http://localhost:3000:

docker run --rm -p 3000:3000 grafana/plugin-builds:a973fdd319a3656ca4a634702c8a8d006c4f4609pre

[bossinc]

This doesn't seem to work when I create constants without quotes, but I don't think we should be modifying template variable values in components. That should be done in applyTemplateVars function in datasource

[aangelisc]

This doesn't seem to work when I create constants without quotes, but I don't think we should be modifying template variable values in components. That should be done in applyTemplateVars function in datasource

Good callout @bossinc, I'll take a look at just implementing this in applyTemplateVariables (and how to handle quoted values).

[javisst]

@aangelisc Do you have an update on this?

[aangelisc]

Hi @javisst, not yet - I'm hoping to get to this next week.