Track user logging in successfully

`config.html` doesn't work because it's loaded before `base.html` loads `main.js` which gives us the `mdn.analytics` object that's required by this code. 

I like @darkwing's idea to put this into a `.js` file.

I propose:

1. Add `data-username="{{ request.user.username }}"` to `templates/base.html`
2. Move this to a `trackSignins` function in `analytics.js`