Automatize CLA signing workflow

[dacap]

Users think that after the Google form is sent, the CLA signing process ends, but actually that is when the process starts. After we receive a new form entry, we have to send the form to sign the document via email with https://www.signnow.com/, but it looks like this is not 100% clear in the steps.

We have to do something about it. One possibility:

• Just put an email address in the Google Form we put one email address + full name + github user name to check then the signed doc
• Send the signnow doc automatically after the Google form is filled (we have to use the signnow API for that)

EDIT: So far the signnow doc is sent automatically when the form is filled 👍

So tasks for the future:

• save the user name into the list of signed CLAs (Google Sheet)
• save/backup the signed CLA in Google Drive

For these actions we have to create a webhook to receive the event when the CLA is signed.

Other tasks to automatize GitHub workflow:

• add a webhook when a PR is created to check if the user has signed the CLA
• label the PR with cla-signed (@aseprite-bot could do this?) or add a comment pointing to the CLA signing process or to the CLA form directly