Made asynchronous upload by URL working, partly. Hid it behind $wgAllowAsyncCopyUploa...

Made asynchronous upload by URL working, partly. Hid it behind $wgAllowAsyncCopyUploads. If there are no errors then everything works expected; the same if there are unrecoverable errors. User intervention to solve warnings is not yet possible, because $_SESSION is not available in runJobs. This also means that async with leavemessage = false is broken.

Other changes:
* Moved verifyPermissions check in ApiUpload down pending r70135 implementation in the API.
* In User::leaveMessage: append message to end of talk page; add a newline before the heading