Link different reports together

[preaction]

We should have a way to link different reports together for various reasons. The most common reason will be "same underlying issue", but another reason could be "caused by upstream issue" or "causes downstream issue".

Once the data is added to the report (cpan-testers/cpantesters-api#33), we need a way to add links. The most common thing to do will be to link together a bunch of reports that are caused by the same issue, so we should create a list of the reports, allow users to select multiple reports, and let them click a button to mark them all as related. The button should pop-up a dialog to enter a comment about why these reports are being related (for example These are all caused by the tests being run during a DST change.)

This will require implementing the mockup for listing the reports for a given dist release (https://github.com/cpan-testers/cpantesters-web/blob/master/share/templates/dist.html.ep). A simple text input form on the beta landing page can get users to the page for their dist.