Fall back to username as coll title on OAuth signup

This uses the given username as the Display Name / Collection Title if a
user doesn't give one -- as might happen when authenticating with
Write.as.

Ref T712

oauth_signup.go

@@ -133,7 +133,7 @@ func (h oauthHandler) validateOauthSignup(r *http.Request) error {
 	}
 	collTitle := r.FormValue(oauthParamAlias)
 	if len(collTitle) == 0 {
-		return impart.HTTPError{Status: http.StatusBadRequest, Message: "Display name is too short."}
+		collTitle = username
 	}
 	password := r.FormValue("password")
 	if len(password) == 0 {