5.6 Account registration

### Problem

We need to be able to register users to this new chat system. For this first task, we want to validate the user email is available, allow them to register it, and confirm via email that they have been successfully created.

The relevant Matrix specs are below:

```
5.6.1 POST /_matrix/client/r0/register
5.6.2 POST /_matrix/client/r0/register/email/requestToken
5.6.8 GET /_matrix/client/r0/register/available
```

### Requirements
* All code should be production level code, whatever that means to you.
* Code should be written in a forked repository, and submitted via a Pull Request to this repo.
* Some sort of permanent storage should be used to save user information. What you use is up to you.

### Resources
https://matrix.org/docs/spec/client_server/r0.6.1#account-registration-and-management

Start by reviewing the Matrix account-registration specification and the three named endpoints: POST /_matrix/client/r0/register, POST /_matrix/client/r0/register/email/requestToken, and GET /_matrix/client/r0/register/available. Identify the existing server entry points and determine how permanent user storage and email confirmation fit the project. Done means users can check email availability, register, and receive confirmation of successful creation.

Written by the indexing model from the issue text.