Bugzilla Keyword Renaming: footprint and mlk

Per bug https://bugzilla.mozilla.org/show_bug.cgi?id=1420543, we are
renaming the footprint and mlk keywords to memory-footprint and
memory-leak, respectively, to make them more discoverable when editing bugs.

The relevant stakeholders have been consulted and approve of the change.

Existing bugs using these keywords will update automatically and you do not
need to edit them.

If you have custom reporting URLs or web apps consuming these keywords,
please update them.

Thank you for your kind attention,

Emma, Bugmaster