Any upload is better than no upload

[duzda]

Basically, if all the uploads get filtered, then the user can't download the app, this PR changes the behavior and if there are no valid uploads left, then it allows the user to choose any of the uploads. How could this be helpful? In my case, I commonly use itch app for game jams, but if the dev doesn't select the appropriate platform, suddenly his game is not downloadable by itch.

This PR changes behavior from this:

To this:

However if dev does specify the platform, then no changes are made and user gets shown only the appropriate ones for his OS.
It might be nice to notify the user about the unknown platform in the itch app, I think I can add that later.

[hsandt]

Very useful for jam games, where devs tend to upload in a hurry, sometimes not using butler and not benefiting from the automated build name detection; and for Renpy games where the "pc" build is for Windows and Linux but that's not obvious from the name, so even butler misses that and cannot automatically tag the game (maybe for Windows, I don't remember, but definitely not Linux).

Still needed to comment on a game page to tell dev to properly tag their builds today.

Workaround: open page in web browser and manually download the build, but more cumbersome (and esp. feels weird as itch app is already a web app)

P.S.: I just realized this PR has no associated issue (#2929 being slightly different), so I have no choice but to comment here although PR thread is usually for comments about the PR itself. If it's a problem I can continue talking on #2929 instead, but really I probably don't have anything more to say at this point.