enhancement env var for extra_template_basedirs

[ndmlny-qs]

This commit add the following three items.

• Updates the method _default_extra_template_basedirs in the module templateexporter.py to search for a new environment variable called NBCONVERT_EXTRA_TEMPLATE_BASEDIRS. This environment variable will be added to the basedirs list if it is found, and it exists.
• Updates the tests to ensure if the new environment variable exists then it is included in the TemplateExporter.template_paths.
• Adds a context manager method that modifies the os.environ dictionary.

Closes #2026

[ndmlny-qs]

@Carreau let me know if you'd like additional items added or any changes or more tests. Thanks!

[Carreau]

I have two worries:

1. extra_template_basedirs is already a convenience config on top of extra_template_paths
2. we're modifying the default value, so if someone also use CLI or config file there will likely be conficts/unexpected behavior.

Let me think about it a bit.

[Carreau]

I've posted a followup in #2026 as I'd like to better understand the use case.
I'd like to avoid one-off patches if we can have a general one, and I think the use case is already covered by how traitlets can be configured, so just checking why and seeing if we can offer a more generic solution.