When creating a group membership request, there is no character
limit on the 'reason' field. This can be potentially be used by
an attacker to create enormous amount of data in the database.
Co-authored-by: Ted Johansson <ted@discourse.org>
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
