BIOS Dialog UX Improvements

This is a tracking issue for BIOS dialog UX improvements:

[x] Add "Sign In" button
[x] Add "Sign Up" button
[x] Add "Sign Out" button
[x] Add join inst BIOS option
[x] Make join inst a default BIOS option instead of enter join code
[x] Make join inst say "Load" instead of "Start" on the continue button
[x] Add a description for join code BIOS options: "enter a join code to load an existing inst"
[x] Display the description for the selected BIOS option below the selection box.

casual-simulation / casualos