Discourse membership requests lack character limit

Discourse is an open-source discussion platform. Prior to version 3.0.1 on the `stable` branch and 3.1.0.beta2 on the `beta` and `tests-passed` branches, when submitting a membership request, there is no character limit for the reason provided with the request. This could potentially allow a user to flood the database with a large amount of data. However it is unlikely this could be used as part of a DoS attack, as the paths reading back the reasons are only available to administrators. Starting in version 3.0.1 on the `stable` branch and 3.1.0.beta2 on the `beta` and `tests-passed` branches, a limit of 280 characters has been introduced for membership requests.