Add maximum length to subject

[sentry-io]

Sentry Issue: APPLICATION-BACKEND-6TF

ProgramLimitExceeded: index row size 3736 exceeds btree version 4 maximum 2704 for index "emails_email_subject_idx"
DETAIL:  Index row references tuple (3883,3) in relation "emails_email".
HINT:  Values larger than 1/3 of a buffer page cannot be indexed.
Consider a function index of an MD5 hash of the value, or use full text indexing.

  File "django/db/backends/utils.py", line 89, in _execute
    return self.cursor.execute(sql, params)

OperationalError: index row size 3736 exceeds btree version 4 maximum 2704 for index "emails_email_subject_idx"
DETAIL:  Index row references tuple (3883,3) in relation "emails_email".
HINT:  Values larger than 1/3 of a buffer page cannot be indexed.
Consider a function index of an MD5 hash of the value, or use full text indexing.

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  File "api/views/emails/routes.py", line 461, in update_email
    instance.save(update_fields=changed_values)
  File "emails/models/email/model.py", line 397, in save
    return super().save(*args, **kwargs)