Refactor roomserver/internal - split perform stuff out (#1380)

- New package `perform` which contains all `Perform` functions
- New package `helpers` which contains helper functions used by both
  perform and query/input functions.
- Perform invite/leave have no idea how to `WriteOutputEvents` and this
  is now returned from `PerformInvite` or `PerformLeave` respectively.

Still to do:
 - RSAPI is fed into the inviter/joiner/leaver - this introduces circular
   logic so will need to be removed.
 - Put query operations in a `query` package.
 - Put input operations (and output) in an `input` package.
 - Factor out helper functions as much as possible, possibly rejigging the
   storage layer in the process.

roomserver/internal/perform/perform_publish.go

@@ -0,0 +1,25 @@
+package perform
+
+import (
+	"context"
+
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/roomserver/storage"
+)
+
+type Publisher struct {
+	DB storage.Database
+}
+
+func (r *Publisher) PerformPublish(
+	ctx context.Context,
+	req *api.PerformPublishRequest,
+	res *api.PerformPublishResponse,
+) {
+	err := r.DB.PublishRoom(ctx, req.RoomID, req.Visibility == "public")
+	if err != nil {
+		res.Error = &api.PerformError{
+			Msg: err.Error(),
+		}
+	}
+}