Check whether 'load' is the correct event

[gavanderhoorn]

Tracing extension execution there may be a chance safelinksfixer is not using the correct event to register its callback on.

Using a more specific event (ie: closer to where we actually want to be invoked) may be more efficient.

Some resources:

• https://developer.mozilla.org/en-US/docs/Web/Events
• https://developer.mozilla.org/en-US/docs/Mozilla/Thunderbird/Thunderbird_extensions/An_overview_of_the_Thunderbird_interface#Message_window
• https://stackoverflow.com/questions/11370903/thunderbird-event-for-viewing-a-message
• https://stackoverflow.com/questions/162057/how-do-you-insert-email-headers-with-a-thunderbird-extension