Add ADMIN_DOMAIN and support for "global" domains

[matt17r]

This PR addresses a number of existing issues. I've tried to do so in a purely additive/optional way. Existing installations should not be impacted unless they change their configuration.

"Admin" domain

• Admin domain (env.ADMIN_DOMAIN) is added as an optional environment variable.
• If set, it's the domain you use to access the admin dashboard through.
• This allows you to run Kutt at a long url (e.g. links.my-company.example.com) while still using a short URL for links (e.g. eg.url), keeping the published links separate from the backend service

"Global" domains

• Admins can add one or more "global" domains.
• Global domains are available to all users (but links must still be unique).
• Addresses Issue: After Admin1 adds "Custom domain" in settings. Then, no other admins or users can select or use the "Custom domain" #861

"Default" domain

• With these changes, env.DEFAULT_DOMAIN can now live up to its name and be the default, without having to go into the "advanced" settings
• Addresses Client: Add option to set default domain #446

Known issues

Homepage/fallback redirect

The DEFAULT_DOMAIN does not support fallback redirects to a homepage. One possible solution would be to set it in the env also (e.g. env.DEFAULT_HOMEPAGE). Not yet implemented in this PR.

Default domain not obvious when admin domain is set

The default domain is not listed on the Admin → Domains page. If the admin domain is set, it's not obvious what the default domain is. In this case, it might be worth appending it to the bottom of the domains table as a static entry or showing it somewhere else on that page?

P.S. Sorry for all the whitespace only changes. My editor automatically removes trailing whitespace whenever I edit a file

[Loocos]

Hello @matt17r,

Thanks for your PR, I was also looking for such a feature.

I would suggest another way, which is to be able to only say SHARE_CUSTOM_DOMAINS=true/false
This way we do not have to add it to the list of domains in the .env

Use case: Agencies that run tracking links for their customers, they need to add a domain for every single customer.
This will require a technical assistance to edit the .env for each domain.

What do you think?

[matt17r]

Good point @Loocos! I've fleshed out the implementation and moved away from .env for global domains. Much simpler to just add an isGlobal flag that admins can set.

I've tested this locally and I think this is now ready for review. It would close off two items from the backlog @poeti8 and I'm pretty sure it works seamlessly with existing installs.

[Loocos]

Awesome @matt17r !
Thanks for doing it, this gonna be very useful!

Can't wait to see it merged 😁