Require all archive URLs to be for TNA

As a mitigation against malicious URLs being entered by malicious users or
attackers, prevent any URL that isn't The National Archives web archive.

All of the existing custom archive URLs I could find were for this domain.

This was one of the outcomes of the penetration test.
Slight tidy to avoid empty exception blocks
Merge pull request #113 from alphagov/archive_urls_must_be_tna

Require all archive URLs to be for TNA