Harmonise UX behaviour where no archive link is available

[tlongers]

The behaviour of the display of source archive links needs tidying up. Currently it works like this:

• Where source:archive_url is populated with a URL, sfm-cms displays the URL in two places: inside the "Archive Link" column of a record's source content block; and, in the "Archive URL" field of the revelant datapoint citations popover.
• If placeholder text that is not a URL (such as "No Archive Available") has been entered in source:archive_url, in the datapoint citation popover sfm-cms will display this as the label for the hyperlink.
• Where there is no value in source:archive_url, sfm-cms will insert the present URL into the hyperlink.

It should work like this:

• The importer should validate source:archive_url as URLs, and log an error where non-URL text is found
• The field label for archive links in the datapoint citation popover and a record's source content block should be the same: "Archive Link".
• In both locations, where there is no valid value in source:archive_url, the text "Unavailable" should be displayed. No hyperlink should be generated.
• The text "Unavailable" should be tagged for localisation.