PrivacyOptions: default usage collection to false

Shifting to an opt-in mechanism instead of opt-out, thus we should start
with a default value of false.
UsageUploadService: use version key

We now pass a version key in our json string. This allows the server to
reject upload attempts from outdated versions if desired (due to
changing schema, permissions, and so on).
UsageUploadService: add status message

We now print a message when the upload begins, which should help explain
why there might be a delay when quitting.
UsageUploadService: get input on welcome

We now show the privacy options on WelcomeEvents, to prompt users to
opt-in if they would like, and to ensure they are prompted when ImageJ
is updated.