Allow customizing the list of default instances

**Is your feature request related to a problem? Please describe.**
When logging in to an account, Voyager currently shows a list of only 3 default instances, with the option to manually type the URL/domain for a custom instance you'd like to use on a per-case basis (see first screenshot).

**Describe the solution you'd like**
What I'd like to see is an option in the settings to customize the list of default instances, so you can permanently add your preferred instances to the list. Lunar currently does this very elegantly (see second screenshot).

In Lunar, you can access this screen from the top level settings menu via an entry called "Manage Instances", but it could certainly also be placed within one of the existing categories in Voyager's settings menu (e.g. "General") if that seems preferable. The advantage of that would mainly be not having to come up with an appropriate icon choice.

Basically, the entire list is editable (meaning you can remove any of the default instances if you'd like), but you can always get the default list back using the "Reset Instances List" button. It probably isn't necessary for the list to also support re-ordering by dragging and dropping if that means a lot of extra work (it's not orderable in Lunar either), since one can always just remove and re-add instances in the order desired.

**Describe alternatives you've considered**
Not giving users a simple and accessible way to add custom instances will likely contribute to increased centralization in the fediverse by driving more traffic to the "big 3", since manually typing a custom instance name gets old rather quick, and people are naturally lazy.

**Additional context**