Otter-grader

* Fixed requirements not found error for R notebooks in Otter Assign
* Removed use of `re` in overriding `Notebook` test directory override in `otter.execute.execute_notebook` by adding `otter.Notebook._test_dir_override`

* Fixed `NoneType` issue in `PluginCollection.generate_report`

* Added ignoreable lines in Otter Assign
* Added error handling for log deserialization per [190](https://github.com/ucbds-infra/otter-grader/issues/190)
* Added plugin data storage per [191](https://github.com/ucbds-infra/otter-grader/issues/191)

* Fixed positional arg count in ``otter.plugins.builtin.GoogleSheetsGradeOverride``
* Fixed working directory in ``otter.plugins.builtin.GoogleSheetsGradeOverride`` during Otter Assign

* Fixed [185](https://github.com/ucbds-infra/otter-grader/issues/185)

* Added the `each` key to `points` in question metadata for Otter Assign