Account registration: Restrict Real Name length

Summary:
Avoid a database exception at user account registration when users enter very long real names by setting a maximum length.
This does not affect existing account data as it is only called in the account registration code.

Fixes T15962

Test Plan: Go to http://phorge.localhost/auth/register/ and enter long values into the "Real Name" field

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: l2dy, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15962

Differential Revision: https://we.phorge.it/D25841