Add basic Matrix puppeting support

May contain bugs.
EDUs from /sync are not yet handled.

database/upgrades/2019-05-23-puppet-custom-mxid-columns.go

@@ -0,0 +1,23 @@
+package upgrades
+
+import (
+	"database/sql"
+)
+
+func init() {
+	upgrades[5] = upgrade{"Add columns to store custom puppet info", func(dialect Dialect, tx *sql.Tx, db *sql.DB) error {
+		_, err := tx.Exec(`ALTER TABLE puppet ADD COLUMN custom_mxid VARCHAR(255)`)
+		if err != nil {
+			return err
+		}
+		_, err = tx.Exec(`ALTER TABLE puppet ADD COLUMN access_token VARCHAR(1023)`)
+		if err != nil {
+			return err
+		}
+		_, err = tx.Exec(`ALTER TABLE puppet ADD COLUMN next_batch VARCHAR(255)`)
+		if err != nil {
+			return err
+		}
+		return nil
+	}}
+}