Users can now provide the URL of a website instead of the url of a RSS
feed via the input field for the RSS source.
This is possible because we are now trying to get and parse the RSS feed
for the provided url as usual, but if this operation fails, we try to
parse the text as html, so that we can check if it contains a
"<link type="application/rss+xml" href="RSS_FEED_URL">" tag. If this is
the case we are using this value to try to get and parse the RSS feed
again. If it fails again we are returning an error as before.
All our forms were submitable via enter except the forms used to add a
new source to a column. This is now changed so that also these forms can
be submitted by pressing enter in a text field.
