Add linting for shell scripts
<!-- Please describe your changes on the following line: -->
This changes tidy to check shell scripts for the proper shebang and
options. It does not check that variables are formatted correctly. It
also adds a check for the MPL 2.0 license in shell scripts.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#12158 (github issue number if applicable).
<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12295)
<!-- Reviewable:end -->
This changes tidy to check shell scripts for the proper shebang and
options. It does not check that variables are formatted correctly. It
also adds a check for the MPL 2.0 license in shell scripts.
This changes the script to pass the `--mime-type` option to `s3cmd`, as
described [here](http://s3tools.org/usage). The MIME type used is
`application/octet-stream`.
Fixes#12153.
The nightly package name includes the date, and so we use a glob to
locate the package file without hard coding a value. However, globbing
will not work with our Buildbot steps setup because we perform word
splitting ourselves and pass an array to Buildbot, which will directly
exec the array instead of passing it to the shell, meaning globbing
does not occur. Instead, add a script to the servo repo where we can
use globbing, and use `shopt -s failglob` to guard against bad globs.
