This just implements a basic sign-in flow, using the new endpoints in #29 :

This brings over several dependencies that are necessary:
- `formik`
- `yep`
Ports over some v1 code to bootstrap it:
- `FormTextField`
- `PasswordField`
- `CoderIcon`
And implements basic sign-in:
Fixes#37Fixes#43
This does not implement it navbar integration (importantly - there is no way to sign out yet, unless you manually delete your `session_token`). I'll do that in the next PR - figured this was big enough to get reviewed.