Implement MagicFolders list view

Closes #31

See merge request !34

... it looks kinda odd where it's at on my mobile.

... the magic-folder list UI design doesn't have any

Thanks Haddock!  Who told you to run at all?

Fix firstRun page CSS I forgot to move over

See merge request !33

Still a bit ugly until CSS nesting lands

https://www.w3.org/TR/css-nesting-1/
https://developer.chrome.com/articles/css-nesting/

View renames: Splash -> FirstRun, Folder -> FileBrowser

See merge request !32

This is more different from the "MagicFolders" view

... since we want to display this if there's no Magic Folders yet.

Utilize Obelisk's router for navigation within our app

See merge request !31

But ghci is perfectly content with this.  :(

it should really be NonEmpty but then we'd have to write a NonEmpty path
encoder, I guess

Actually they were mostly fine before and the problem was really in the file
browser ... oh well.  This is fine too.

Readme: Add links to milestone and issue overview

See merge request !29

Add tahoe-great-black-swamp as a dependency

Closes #14

See merge request !30

The version from nixpkgs is missing conduit, conduit-extra, and resourcet
dependencies, probably because of the complexity of the upstream .cabal file -
or maybe because of the use of CPP features in the implementation (I'm not
sure which).

Here's a cabal2nix-generated package that has been hand-edited to include the
missing build dependencies.