Allow user names up to 26 characters long, in line with Android.

See https://github.com/oxen-io/session-android/blob/master/libsession/src/main/java/org/session/libsession/utilities/SSKEnvironment.kt#L29

ts/components/registration/RegistrationStages.tsx

@@ -16,7 +16,7 @@ import {
 } from '../../util/accountManager';
 import { fromHex } from '../../session/utils/String';
 
-export const MAX_USERNAME_LENGTH = 20;
+export const MAX_USERNAME_LENGTH = 26;
 // tslint:disable: use-simple-attributes
 
 export async function resetRegistration() {