Add an option to configure allowed/block instances

**Is your feature request related to a problem? Please describe.**
Not a problem _per se_.

**Describe the solution you'd like**
I'd like to limit my deployment of Phanpy to be limited to instances I allow (or blocks specific ones), to deter people from using my server to access instances I consider bad.

**Describe alternatives you've considered**
Soft-forking Phanpy to add this myself, but I think this would benefit more people.

**Additional context**
I know this can always be avoided by modifying code on the browser, but it's at least another layer of annoyance, more symbolic that anything else, and seems relatively easy to implement? I can try to help :)