Improve alignment of old/new fields in some edit diffs

Pull request here: https://bitbucket.org/metabrainz/musicbrainz-server/pull-request/614/mbs-7036-improve-alignment-of-old-new/diff

Changes the colspan of edit display tables that mix one-column/two-column table rows.

Examples demonstrating a readability improvement:

• Edit area: https://musicbrainz.org/edit/22694954 http://intgr.mbsandbox.org/edit/22694954
• Edit cover art: https://musicbrainz.org/edit/23241103 http://intgr.mbsandbox.org/edit/23241103
• Edit label: https://musicbrainz.org/edit/23205451 http://intgr.mbsandbox.org/edit/23205451
• Edit recording: https://musicbrainz.org/edit/23241830 http://intgr.mbsandbox.org/edit/23241830
• Edit release group: https://musicbrainz.org/edit/23237236 http://intgr.mbsandbox.org/edit/23237236
• Edit work: https://musicbrainz.org/edit/23233621 http://intgr.mbsandbox.org/edit/23233621