Otter-grader

* Fixed [389](https://github.com/ucbds-infra/otter-grader/issues/389) by closing file handles in Otter Grade before removing them
* Fixed R image builds by setting Conda `channel_priority` to `strict` in R `setup.sh` files per [386](https://github.com/ucbds-infra/otter-grader/issues/386)
* Fix loading docker images after building
* Added the `--ext` option to Otter Grade per [386](https://github.com/ucbds-infra/otter-grader/issues/386)

* Specify UTF-8 encoding in all `open` calls used for reading JSON for Windows compatibility per [380](https://github.com/ucbds-infra/otter-grader/issues/380)
* Fixed incorrect prompt substitution in R notebooks and Rmd assignments

* Added `libgomp` to R `environment.yml`

* Added a tool to convert Assign v0-formatted notebooks to v1 format
* Added `xeCJK` to the LaTeX exporter template and Docker images
* Added PDF generation and submission in R per [302](https://github.com/ucbds-infra/otter-grader/issues/302)
* Updated intercell seeding in R per [302](https://github.com/ucbds-infra/otter-grader/issues/302)
* Fixed bug in test case point values for Assign R assignments per [360](https://github.com/ucbds-infra/otter-grader/issues/360)
* Enabled `solutions_pdf` and `template_pdf` in Assign R assignments per [364](https://github.com/ucbds-infra/otter-grader/issues/364)
* Added options to limit execution time of grading and permit network access
* Refactored R submission reformatting per [369](https://github.com/ucbds-infra/otter-grader/issues/369)
* Added export cells for R notebooks in Otter Assign per [369](https://github.com/ucbds-infra/otter-grader/issues/369)
* Updated the default version of Ottr to v1.1.1
* Changed prompts in R notebooks to match Rmd documents
* Added filtering of notebook cells with syntax errors during R notebook execution
* Enured that "empty" tokens are ignored in Otter Generate per [361](https://github.com/ucbds-infra/otter-grader/issues/361)
* Prevented save text when `export_cell: force_save: true` is specified in Otter Assign assignment metadata per [332](https://github.com/ucbds-infra/otter-grader/issues/332)

* Refactored Otter's execution internals
* Added intercell seeding via seed variables per [346](https://github.com/ucbds-infra/otter-grader/issues/346)
* Made Otter Run compatible with ZIP submissions
* Fixed argument bug in Otter Run per [349](https://github.com/ucbds-infra/otter-grader/issues/349)