Styling updates

* Updated Bootstrap version.
* Added jQuery and Bootstrap.js.
* Added navbar and moved login stuff there.
* Renamed `hello` template into `homepage` to make things less confusing.
`@grid-float-breakpoint` is a breakpoint where Bootstrap layout switches between "mobile" and "desktop" layouts.

So in the first case (desktop layout) all flashes are pulled to the right side of the page:
![Desktop layout](https://i.imgur.com/HHifykB.png).

When page width reaches breakpoint, it displays flashes right under navbar. They have full width in this case to look better.
![Mobile layout](https://i.imgur.com/Cpzjnf6.png)