CI notification verification and preview

At the moment I'm still testing each new notification by hand. To streamline the notification feed, we need a travis script that will:

- verify that all values are appropriate
- type is either announcement, new-feature, blog, or empty
- link is either hypertext protocol or, if starting with `orange://`, is defined in master
- start/end are valid
- ...
- check that icon is present in Orange3 master (possibly also previous releases, informing which releases it's present in),
- generate the new notification and comment on the PR with a screenshot.

Start by locating the Travis configuration and the notification feed data, then inspect how Orange3 master exposes notification types, links, dates, and icons. Done means CI validates the listed fields, checks icon availability, generates the notification, and comments on the PR with a screenshot.

Written by the indexing model from the issue text.