Allow package uploaders to remove themselves when other uploaders exist

[sigurdm]

Allow uploaders to remove themselves from a package when there is more than one uploader:

• Remove the selfRemovalNotAllowed check in PackageBackend.removeUploader (removing the last uploader remains forbidden).
• Hide the remove uploader (×) button column on the package admin page when there is only a single uploader.
• Show an explicit self-removal warning in the confirmation dialog when an uploader removes their own account, and redirect to the package page afterwards.

Fixes #2688