Support Serializing Emails With Alternatives In Django 5.2

[dchukhin]

When sending an email with alternatives in Django 5.2 with the RedisCache backend, we encounter the following error in cache.set(MAIL_TOOLBAR_CACHE_KEY, outbox, MAIL_TOOLBAR_TTL) (in mail_panel/utils.py):

_pickle.PicklingError: Can't pickle <class 'django.core.mail.message.Alternative'>: attribute lookup Alternative on django.core.mail.message failed

This pull request adds some logic that turns an outbox's emails into serializable dictionaries prior to putting the outbox into the cache, and turns the serialized data back into MailToolbarBackendEmail objects when pulling the outbox from the cache.