feat: Initial Login flow (#42)

This just implements a basic sign-in flow, using the new endpoints in #29 :
![2022-01-20 12 35 30](https://user-images.githubusercontent.com/88213859/150418044-85900d1f-8890-4c60-baae-234342de71fa.gif)

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 #37 
Fixes #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.

_jest/setupTests.ts

@@ -0,0 +1,7 @@
+/**
+ * Global setup for our Jest tests
+ */
+
+// Set up 'next-router-mock' to with our front-end tests:
+// https://github.com/scottrippey/next-router-mock#quick-start
+jest.mock("next/router", () => require("next-router-mock"))