Generic layout/UX improvements

[filp]

For my own personal use, and to share with those close to me, I've made some layout and presentation changes which I've bundled into this PR.

The commit messages are pretty much self-descriptive, but here's a run-down of what I've done:

• Added a simple error page
• Fixed an XSS injection in the error page
• Cleaned-up the HTML a bit
• Updated bootstrap.css to a minified version of the latest bootstrap
• In the process, I added support for a basic mobile layout
• Replaced the fall-back index view with a more generic and simpler one
• etc

The actual final presentation, what the user sees, is hardly changed, except for the

Mobile layout, making use of bootstrap's fluid grid:

[victorstanciu]

Awesome, I've merged your changes, thanks a lot!