Feed Fetching, timestamps and more

[Grotax]

We added back the feature to use timestamps in our requests to feeds in #2119.
This was also the case in earlier versions of News but we faced the issue that some Feeds would have missing updates, since then the code has changed a lot and so did feed-io, therefore the current implementation is not exactly the same.

For the first fetching of a feed during creation we now use a timestamp "one year ago" to make sure that we get all items.

We could face the problem that some feeds are badly configured and that they do not reply properly and news ending up with missing items.

A possible workaround would be to fetch feeds once a day for example also with the "one year ago" timestamp.
Due to the architecture of the background jobs I don't think it makes sense to actually have a second job.

We could add a new attribute to the feed, indicating when the last "full fetch" was done. Based on a timestamp.
If that timestamp is older than 24 hours, instead of using the httpLastModified we send "one year ago".

I'm not sure if this is a good idea but I guess the better alternative to not using timestamps at all.