Backend image rendering as plugin (#11966)

* rendering: headless chrome progress

* renderer: minor change

* grpc: version hell

* updated grpc libs

* wip: minor progess

* rendering: new image rendering plugin is starting to work

* feat: now phantomjs works as well and updated alerting to use new rendering service

* refactor: renamed renderer package and service to rendering to make renderer name less confusing (rendering is internal service that handles the renderer plugin now)

* rendering: now render key is passed and render auth is working in plugin mode

* removed unneeded lines from gitignore

* rendering: now plugin mode supports waiting for all panels to complete rendering

* fix: LastSeenAt fix for render calls, was not set which causes a lot of updates to Last Seen at during rendering, this should fix sqlite db locked issues in seen in previous releases

* change: changed render tz url parameter to use proper timezone name as chrome does not handle UTC offset TZ values

* fix: another update to tz param generation

* renderer: added http mode to renderer service, new ini setting [rendering] server_url

pkg/plugins/renderer_plugin.go

@@ -0,0 +1,22 @@
+package plugins
+
+import "encoding/json"
+
+type RendererPlugin struct {
+	PluginBase
+
+	Executable string `json:"executable,omitempty"`
+}
+
+func (r *RendererPlugin) Load(decoder *json.Decoder, pluginDir string) error {
+	if err := decoder.Decode(&r); err != nil {
+		return err
+	}
+
+	if err := r.registerPlugin(pluginDir); err != nil {
+		return err
+	}
+
+	Renderer = r
+	return nil
+}