Add *optional* email-based password reset flow

[monorkin]

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 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

[milos-dukic]

Hey @monorkin I was wondering, this email-based password reset flow, should it be on login screen like:

And then of course a new form where you enter your email etc.

Thx.

[monorkin]

@milos-dukic yes, that's the idea.
If SMTP is configured, a forgot password link should appear and ask you for your email.
We envisioned it to work in the same way that the Rails authentication generator does it.

[milos-dukic]

PR #100 created