Add support for WEB_DOMAIN in setup wizard.

[qassim]

Hi all,

After having used the DigitalOcean application to deploy my mastodon instance, I had a bit of trouble due to wanting to host the web application on a different domain than my handle domain (e.g. host on mastodon.qassim.uk but my handle be qassim@qassim.uk)

I had a bit of back-and-forth bodging the install scripts to make it work

So I thought I'd propose a change to the installation wizard to allow for configuring of the WEB_DOMAIN if people want to use it. I understand this can be considered a bit more of an advanced option that we might not want to push users towards, but as I detailed in my blog post above - I made the mistake of initially setting up my mastodon instance in a way I wasn't happy with partly as a result of just following the wizard and not realising and had to come back later on and rebuild and migrate.

I have:

• Built and deployed the images in digitalocean via packer to test.
• Tested both paths (LOCAL_DOMAIN only and WEB_DOMAIN)