Redesign attester pages

Change style in homepage and status pages.

The homepage now displays configuration information while the status provides more details on connected browsers and remaining tasks.
This closes #6 as well
Closes #25
Adding try/catch in slave handlers to avoid crashes.
Preventing a new history entry to be added for each test.

A new history entry is no longer created for each test. This is done by
creating a new iframe DOM element for each test (after the one for the
previous test is removed).