Provide a login form on every page

- Uses native bootstrap-dropdown
- failures redirect to ```/login``` with
- to get bootstrap dropdowns to work I had to upgrade bootstrap to v2.2.2
- successful logins redirect to the page which the user attempted to login from

I did this in three parts to more clearly show the changes:
1. upgrade bootstrap
2. add the code for the login form
3. handle the redirect when logging in from not ```/login```