Add API call to get linkding version

[bachya]

Now that I've created aiolinkding and linkding-cli, I'd like to be able to enforce minimum linkding versions (so that I can notify users if one of those packages requires a newer version than what they have). To that end, it'd be useful to have a simple API call that gets the currently running version.

I attempted a PR for this, but apologies, Django REST Framework is too opaque for my simple mind to understand. 😂

[kencx]

I can probably get started on this.

[sissbruecker]

Closing as resolved with https://github.com/sissbruecker/linkding/pull/392