Motivation:

Some server-side problems are quickly fixed by retrying the request,
while other problems take longer to recover.  Using a fixed duration as
a delay between attempts makes it hard to reconcile these different
failure modes.

Modification:

Add a progressive timeout strategy.  The delay between attempts now
scales linearly between successive attempts.

As a special case, the retry strategy is different if the server has not
provided any useful response.  In this case, a fixed delay is used.

Also, catch more problematic scenarios and handle them with the same
retry strategy.

Result:

More robust handling of server-side errors, by retrying.

The recovery logic when faced with an error querying the identifiers is
currently broken, as it returns nil instead of a hash.

Observed transitory failures with GET requests; for example, timeouts.
We would like the harvesting routine to be robust against such temporary
failures.

Remove redundant whitespace and change some spaces to use non-breaking
space instead.

Calculate total OAI-PMH.

Move both totals into the table

The display now shows the number of datasets available
from the endpoint.

Commit fe68e8d5 fixed the script for checking whether the PaN search
API is working.  Unfortunately it failed to fix the broken links on the
webpage.

This patch fixes those links on the webpage

The `check_search_api.rb` script contained a bug because of the
following issue:

    https://github.com/panosc-eu/search-api/issues/75

This updates the script and rectifies the status of institutes.

Don't display anything in Open data repository column if a facility
doesn't have the corresponding link.

For OAI-PMH and PaN-Search endpoints, only show a red cell if
status is Error.

The AMBCAT contract is now signed.  Add link to OSCARS web-page
fix corruption in JSON.