Add manual runner removal

Runners can now be manually removed using the CLI. Some restrictions apply:

  * A runner must be idle in github. Github will not allow us to remove a runner
that is running a workflow.
  * The runner status must be "running"

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

database/sql/models.go

@@ -116,6 +116,7 @@ type Instance struct {
 
 	ProviderID    *string `gorm:"uniqueIndex"`
 	Name          string  `gorm:"uniqueIndex"`
+	AgentID       int64
 	OSType        config.OSType
 	OSArch        config.OSArch
 	OSName        string