Add a nag for missing source tag on changeset/upload

[olejorgenb]

Depressingly few people employ the source tag, at least in my area.

Maybe a nag, or at least having a dedicated textbox in the upload dialog, could help? If not for the purpose of forcing people to use the tag, then for helping people that do remember adding it.

When a changeset is closed there is no way back, and then you are left with tagging all elements, which is annoying, and have slightly different semantics. (ie. for changed, not new elements)