Remove YAML dependency

Apparently the YAML gem is not required. Actually, the opposite was the
case and the dependencies could not be satisfied which lead to an error
when trying to start the Rails server.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Change metrics overview to a HTML form

Instead of using links I am currently reworking the metric computation
panel into a HTML form. At the time being this is necessary as it
depends on the user input on which repository the metrics should be
applied.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>