show diff against base table when sending e-mail alerts for scheduled changes

[bhearsum]

Currently we send mail such as:
Row to be inserted:

{'base_alias': 'firefox-release',
 'base_backgroundRate': 100,
 'base_buildID': None,
 'base_buildTarget': None,
 'base_channel': 'release',
 'base_comment': 'default release rule updated by buildbot, DO NOT DELETE',
 'base_data_version': 147,
 'base_distVersion': None,
 'base_distribution': None,
 'base_fallbackMapping': None,
 'base_headerArchitecture': None,
 'base_locale': None,
 'base_mapping': 'Firefox-54.0-build3-whatsnew',
 'base_osVersion': None,
 'base_priority': 90,
 'base_product': 'Firefox',
 'base_rule_id': 145,
 'base_systemCapabilities': None,
 'base_update_type': 'minor',
 'base_version': None,
 'change_type': 'update',
 'csrf_token': '1498007702##fe6927e9a30927fb3b0baebef3ea7f86bf04e314',
 'data_version': 1,
 'scheduled_by': '[email protected]'}

Unfortunately, scheduled changes are not terribly useful without context. The most useful thing to know is "what is this scheduled change going to change". In the above case, backgroundRate and fallbackMapping were different vs. the base, so that's what should've been highlighted.

Can be closed if we end up killing them (see #1071).

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1375010)