Make Latest activity information available via API

[MikkCZ]

For Pontoon Add-on to mimic the team dashboard, I am parsing the latest team activity (user's name + date in ISO format) from the list https://pontoon.mozilla.org/teams/. It would be very nice to be able to get rid of parsing of the HTML and add this information to the API instead into Locale, but potentially also Project and ProjectLocale for the sake of API completeness.

[mathjazz]

Note: we use GH labels instead of [labels] in the title.