Otter-grader

* Fixed bug in Otter Assign format v1 for closing exports when the last question is manually-graded
* Removed cell IDs in Otter Assign output notebooks per [340](https://github.com/ucbds-infra/otter-grader/issues/340)
* Changed submission zip download link to use the HTML `download` attribute per [339](https://github.com/ucbds-infra/otter-grader/issues/339)

* Fix some minor bugs using docker for grading
* Added support for test config blocks in R assignments
* Fixed use of `autograder_files` in Otter Assign
* Removed the `generate: pdfs` key from Otter Assign assignment configurations
* Converted from Docker-py to Python on Whales for containerized grading

* Made `otter.Notebook.check_all` compatible with metadata tests
* Refactored logging in `otter.Notebook`
* Fixed parsing of Windows newlines in Otter Assign

* Updated the default version of Ottr to v1.0.0
* Ensured that metadata tests for R notebooks are disabled

* Added Colab support to `otter.Notebook` by disabling methods that require a notebook path and ensuring that a tests directory is present
* Added Otter Assign format v1
* Added `FutureWarning` for Otter Assign format v0
* Default-disabled separate test files in Otter Assign v1 format
* Fixed bug in Otter Assign for R causing point values in test files not to render
* Added the `check_cell` key to question metadata in Otter Assign
* Fixed installation of `stringi` in Gradescope R build per [259](https://github.com/ucbds-infra/otter-grader/issues/259)
* Added support for Ottr v1.0.0.b0
* Converted CLI to `click` from `argparse`
* Converted documentation source from Markdown to RST, removing use of `recommonmark`
* Added token argument to Otter Generate and Otter Assign
* Implemented submission zip validation against local test cases in `otter.Notebook`
* Removed metadata files for Otter Grade
* Added flags for disabling file auto-inclusion in Otter Generate

* Added student-visible error reporting in Gradescope results