Added upload of samples into existing project.

Added UploadSamplesMixin, and use it for HTML POST with /project/{projectid}/upload in ProjectUpload, and for API POST with /api/v1/project/{projectid} in ApiProject.

Got rid of obsolete upload.py file.
Aha! No. That page is obsoleted by the new samples upload feature. I actually deleted that entire file upload.py in the current commit.