Add sendMessage() mutation

Add a GraphQL mutation that will be used on the Contact Us page and Send Feedback modal dialog. For example:

```graphql
mutation {
sendMessage(input: { type: FEEDBACK, message: "Hello world!", email: "user@example.com" })
}
```

#### See
* `src/mutations/SendMessage.js`
* `src/server/mutations/contact.js`

Bonus points for making it work with Firebase reCaptcha.

Start by reading src/mutations/SendMessage.js and src/server/mutations/contact.js to understand the existing mutation patterns and the Contact Us or feedback entry points. Implement the sendMessage GraphQL mutation so the example input is accepted and handled; Firebase reCaptcha support is an optional bonus.

Written by the indexing model from the issue text.