Enhancement/Verification emails on Azure

[c-w]

In #250, @guillim introduced the ability for users to self-service sign-up for new Doccano accounts. This pull request ensures that the sign-up functionality works out-of-the-box in the Azure deployment which required a few changes:

1. Enabling configuring the email backend settings via environment variables so that deployments can turn on the feature without requiring a code change.
2. Removing hard-coded HTTP scheme the activation link sent in activation email so that the link also works on deployments that use HTTPS.
3. Adding a Sendgrid resource to the Azure deployment. By default this uses the free tier which covers 25,000 emails per month (see pricing).

Additionally, the wording of the account activation email has been slightly changed to ensure that users know that the email comes from Doccano and we now default to the console email backend to prevent crashes when running locally.