Add option to set the default feed

[e-caste]

On the website there are 3 feed options:

1. Home (which is the only one available in the app as a feed)
2. Local server (accessible via the app but through the search tab which in my opinion is a bit hidden and counterintuitive)
3. Fediverse (this option is completely unavailable in the app, but I believe it should be the most important one since it allows to emulate a "Twitter-like" view and see every update that happens on the fediverse all in one place. Custom Reddit-like rankings (hot, new, popular, etc.) would be a nice addition but I know that I should open another issue in the Mastodon server repo for that)

I'd like to be able to set the default feed, and maybe also easily switch between them quickly (e.g. multi-select in the top bar, Apollo for Reddit does something similar).