Add focal channel for development

[eloquence]

As part of freedomofpress/securedrop#4768 (Focal epic) we'll need to be able to upload and serve Focal packages via apt-test.freedom.press. This issue is intended to track the required changes to this repo's configuration.

[conorsch]

The only change required should be adding a new block to repo/conf/distributions with the focal block. Thereafter, packages can be added to core/focal/ and it'll all Just Work™. At least, that's the plan! 😄