Add new Whitehall::BrokenLinkReporter

This version runs a LinksReport for each public edition and generates
CSV files listing those editions that have broken links, organised by
their lead organisation.
Cleanup old BadLinkReporter code
Update language around broken links

The notifier is sending "broken link" reports not "bad link" reports.
Rake task to generate/send broken link reports
Handle documents with no lead organisation

A Speech that has a person_override set do not have to have a lead
organisation. This updates the BrokenLinkReporter to handle this,
falling back to the first organisation available.
Handle documents with no organisations

It's technically possible for a document to be associated with no
organisations. This handles that case, putting the results from the
broken link checks into a separate CSV file.
Prevent libcurl connection resuse

The broken link reporter becomes unstuck after a certain number of
requests and starts reporting unresolvable host name for all links. Some
Google-ing suggests this is caused by connection reuse by libcurl.
Connection pooling is probably not a good idea with this volumn of links
and domains to check.
Add exception reporting to broken link reporter

So that we are alerted when the rake task falls over.
Timestamp broken link reports filename
Handle SupportingPage in BrokenLinkReporter
Modify language around organisations

The method name #lead_organisation is making less sense now as many of
the documents do not have a "lead organisation".
Increase links_reports.links limit to MEDIUMTEXT

The previous limit was too small to cope with the volumn of links on
some documents. This ups the limit of the column to that of a
MEDIUMTEXT.
BrokenLinkReporter handles CorporateInformationPage

These don't have a lead_organisation method either...
Include public timestamp in broken link report

This will help prioritise documents to fix, e.g. broken links in very
old news articles are less urgent than recent publications or guides.
Merge pull request #1587 from alphagov/rework-broken-link-reporter

Rework broken link reporter