Support Grafana 7

[corpglory-dev]

Fixes https://github.com/chartwerk/grafana-chartwerk-panel/issues/31

Note: Panel-wide timerange is not supported in Grafana 7 because variableSrv.addVariable function is broken:

Changes:

• update @grafana/ui, @grafana/data and @grafana/runtime deps to 7.0.3
• import { TimePicker } from '@grafana/ui' -> import { TimeRangePicker } from '@grafana/ui'
• check Grafana version on plugin load:
  • if version < 6.6.1 - throw
  • if version >= 7.0.0 - do not use panel-wide timerange
• update dist