Remove manual linking to Grafana Explore page

Eventually plan on moving to Mimir's Alertmanager. Once we've done this, we can use the build in template functions such as queryFromGeneratorURL or grafanaExploreURL to link to the Grafana Explore page and remove the manual parsing of the query within the GeneratorURL that we currently do. The manual parsing we do in Slackline will likely have to stay, unless Mimir's Alertmanager also sends these in the alert payload that Slackline receives.