Start searching when hitting enter in user/repo input
Show that the stacktrace is being linked

Before, it was impossible to know if something was actually happening;
now linkStacktrace will replace the input with “working…” while it’s
working.
Put everything in a <form> again
Rephrase introduction paragraph a bit
Add bootstrap.css

Doesn’t look much better right now, I’ll expand on it in later commits
Make it much prettier with bootstrap.css

Adding a few wrapper elements and classes does wonders, it’s really
quite pretty now :)
Improve / add page header

Wraps the introductory paragraph in a div.page-header, and adds a h1
before it. To make all this still fit on the screen, I had to reduce the
default rows of the stacktrace. This shouldn’t be a problem as desktop
browsers (i. e., where you have the screen real estate) allow you to
resize textareas.
Add “Fork me on GitHub” button

Yellow variant, top right. From
https://github.com/blog/273-github-ribbons.
Add request header for GitHub API version
Validate user/repo on input

Currently validates if the user or user/repo matches GitHub’s naming
rules (as inferred from trial and error; couldn’t find any official
resources with the exact rules). Does *not* validate if the user or repo
actually exists – that might be difficult to do responsively.