Replace uploaddir with datadir

[strugee]

As discussed in #1261, deprecate uploaddir and replace it with datadir and uploadsEnabled. Anyone who hates the new names, speak now or forever hold your peace.

[strugee]

@evanp if you have time I'd love a review given that this is semver-major.

[evanp]

I HATE THE NEW NAMES.

[strugee]

@evanp can't tell if you're kidding or not? We can change them but it'd mean another semver-major release

[evanp]

Mostly kidding. I feel like dealing with the deprecation was a pain for me as an admin, and since we don't yet use datadir for anything else except uploads, it wasn't clear what the value was for me as an admin.

I can live with it now, but maybe we could just show a warning if 'uploaddir' is set, instead of aborting?

[strugee]

@evanp ah, okay haha!

Your logic totally makes sense - originally I'd planned to merge some stuff that made use of the change in the same cycle, but ran out of time. I'm uncomfortable doing automatic migrations like this in core (just because it's dangerous and may leave the node broken with no warning) but in the future I'll try to provide better automation, and I'll be uploading a script to perform this that admins can run instead of doing it themselves.

That should alleviate the need to warn instead of abort, which I'd rather not do since 3.0 has been out for a while already.