[#72985526] Extract basic upload dialog

 - Extract basic structure of WorkspaceFileImport into an UploadDialog
[#72985526] User can upload data from hdfs browser

 - User can select a local file and initiate an import to hdfs.
 - If a name conflict is detected, user must provide a unique name in order to
   initiate the import.
 - File is imported asycronously by a worker
[#72985526] Be better at reusing css

 - Refactor css for the upload_widget template
 - Makes its style consistent on the workflow import dialog