Rubric template

Here is what deep and I have done for this MR:

Fixes #3879 (closed), #3810 (closed), #3755 (closed)

• Rubric template can be downloaded through modal form UI:

• The template only contains meaningful columns and some sample entries:

• Upload rubric is made to be compatible with the template

• Extra validations, previously some errors from upload like missing value col or question_index out of range could cause yellow screen of death. This MR patches that

• Make error messages look prettier eg: (otherwise would print rather nasty error msg: Error: {'text': [ErrorDetail(string='This field may not be blank.', code='blank')]})

• Accept blank fields when appropriate (#3810 (closed))