Add *optional* email-based password reset flow

On large or public instances having to manage dozens password reset requests daily can become tedious.
For those cases we'd like to introduce an email-based password reset flow.

This flow should be completely optional, and would only kick in if the instance has email configured.
For sending the emails we can use [ActionMailer](https://guides.rubyonrails.org/action_mailer_basics.html) and to configure it we can use regular SMTP credentials - this will give us a wide range of transactional email providers that people can use with Campfire.

The goal is to provide only password resets for now

Start by locating the existing password reset flow and the instance email configuration, then read the linked ActionMailer guidance for SMTP setup. Done means password resets can optionally be sent by email when email is configured, remain unavailable through this flow otherwise, and do not expand beyond password resets.

Written by the indexing model from the issue text.