Fix SQLite deadlock when creating user

This avoids reading from the database after a transaction has been
started in CreateUser(), fixing the deadlock that occurred before.

Closes #53

database.go

@@ -155,16 +155,16 @@ func (db *datastore) dateSub(l int, unit string) string {
 }
 
 func (db *datastore) CreateUser(u *User, collectionTitle string) error {
+	if db.PostIDExists(u.Username) {
+		return impart.HTTPError{http.StatusConflict, "Invalid collection name."}
+	}
+
 	// New users get a `users` and `collections` row.
 	t, err := db.Begin()
 	if err != nil {
 		return err
 	}
 
-	if db.PostIDExists(u.Username) {
-		return impart.HTTPError{http.StatusConflict, "Invalid collection name."}
-	}
-
 	// 1. Add to `users` table
 	// NOTE: Assumes User's Password is already hashed!
 	res, err := t.Exec("INSERT INTO users (username, password, email) VALUES (?, ?, ?)", u.Username, u.HashedPass, u.Email)