Parallelize HTTP header requests a bit

I have parallelized the HTTP header requests of the accuracy metric
implementation a bit by using the Hydra queue provided by Typhoeus.
Currently, however, only resource requests per dataset are executed
in parallel. I am looking forward to parallelize all resource requests.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add log messages and timeout

I have added some log messages for my convenient and a (connection)
timeout for the accuracy metric HTTP header requests.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Remove repository submit button temporarily

I just noticed that I do not need the repository submit button on the
metrics dashboard. After all the current repository is read dynamically
with JavaScript. I will add it a later point when the metric scores are
loaded from the database.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Create accuracy metric statistic page

In order to gain more knowledge about the metadata characteristic of a
certain repository I will start to add additional sites. I start here by
adding a page which is going to visualize the format distribution.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Implement bar chart for format distribution

I have added the implementation for a bar char visualizing the format
distribution among the repositories. The bar char is ought to be
sortable as well, that, however does not work yet.

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