Allow tailoring of the api key signup e-mail message

[GUI]

When a user signs up, they receive a confirmation e-mail with the api key in it. The body of this message should be configurable so that when the api key signup form is embedded on another site, they can tailor the message how they want. We allow this type of customization on the web signup page (for example, the message can contain a different example URL or a different message body), so we need to offer similar customization for the e-mail message (and maybe use the same parameters used to customize the webpage?).

This came up with the Auctions API embedding the signup page, so see here for a bit more context: GSA/auctions_api#12 (comment)

[GUI]

The new options are documented in the updated snippet here: https://github.com/18F/api.data.gov/wiki/User-Manual:-Agencies#embedding-the-api-key-signup-form-on-your-own-documentation-site (all of the new "OPTIONAL" settings).

In general, the e-mail messages should be improved for all of our existing users without them having to do anything. The custom exampleApiUrl value that agencies were already setting in their embedded forms will automatically be the URL used in the e-mails now. In addition to that improvement, there are new options to further tailor and brand the confirmation page and welcome e-mail message (contactUrl, sendWelcomeEmail, siteName, and emailFromName). I've also belatedly documented a previous option we had (signupConfirmationMessage).