Add-on store: check SHA256 when downloading add-on (#14847)
Part of #13985
Summary of the issue:
The add-on store should refuse to install an add-on with an invalid checksum.
Description of user facing changes
The user is warned if an add-on is downloaded, and the checksum doesn't match the add-on store.
The user is told that the add-on is unsafe, the add-on download is deleted and the install is abandoned.
Description of development approach
Added sha256 function
Testing strategy:
Manually tested by changing the checksum function to output an invalid checksum e.g. "foo" for all add-ons.
Ensured a warning is disabled when following the test plan steps to an install an add-on
