This is a translation of services/alerting/conditions. Main Changes:
- Work with types in SSE (series/number) which are data frames (no more null.Float).
- The query part from alerting/conditions is handled by SSE logic
- Convey / simplejson removed.
- Time range no longer part of "query" in the condition
* comes from grafana/gel-app
* remove transform plugin code
* move __expr__ and -100 constants to expr pkg
* set OrgID on request plugin context
* use gtime for resample duration
* in resample, rename "rule" to "window", use gtime for duration, parse duration before exec
* remove gel entry from plugins-bundled/external.json
which creates an empty array for plugins
