Add support for Thunderbird 60
See https://wiki.mozilla.org/Thunderbird/Add-ons_Guide_57 for changes relevant to Thunderbird 60. This commit also bumps up the minimum supported version to Thunderbird 52.
See https://wiki.mozilla.org/Thunderbird/Add-ons_Guide_57 for changes relevant to Thunderbird 60. This commit also bumps up the minimum supported version to Thunderbird 52.