[ci] move trusty app tests to nightly job and use circle ci branch filtering logic

[redshiftzero]

Status

Ready for review

Description of Changes

This PR makes two improvements to reduce total CI time:

• Beginning with 0.12.0, news organizations will be migrating to Xenial, and all new installs will be on Xenial. Thus, we should move the Trusty app test job to a nightly job, and monitor it for failures until Trusty is deprecated in the coming months. This reduces total CI time per PR, as the trusty job took 20+ minutes. Related to Make Xenial development/CI environments the default #4155.
• It turns out that CircleCI enables filtering by branches now, which we can use to replace our custom logic for avoiding unnecessary jobs on docs- branches, currently used for the staging job only. The app-tests were also being run for docs- branches, so we can use the same CircleCI branch filtering logic to easily exclude the staging/application test jobs run for docs- branches. Indeed, previously jobs would be kicked off and then would rapidly pass, but with the CircleCI branch filtering, jobs won't even get kicked off, so this should reduce build pileups. This should also close Docs-only PRs from forks still run staging env in CI #3699.

Note: We'll need to disable the trusty app test job as a required build to merge, holding off on that until we agree

Testing

Verify that the test build here (branch docs-ci-reduce-jobs) only ran two jobs:

Deployment

CI only

Checklist

CI only

[codecov-io]

Codecov Report

Merging #4158 into develop will decrease coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #4158      +/-   ##
===========================================
- Coverage    84.79%   84.72%   -0.08%     
===========================================
  Files           43       43              
  Lines         2782     2782              
  Branches       303      303              
===========================================
- Hits          2359     2357       -2     
- Misses         355      356       +1     
- Partials        68       69       +1

Impacted Files	Coverage Δ
securedrop/securedrop/crypto_util.py	94.73% <0%> (-1.76%)	⬇️

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[emkll]

Should we also run Xenial infra tests on each PR instead of the default (Trusty) tests?

[redshiftzero]

Once this is approved, I'll remove the requirement for ci/circleci: staging-test-with-rebase and ci/circleci: trusty-app-tests in favor of the corresponding Xenial jobs

[emkll]

Thanks @redshiftzero these changes look good to me. I propose we merge these changes after 0.12.0 release to avoid any side-effects/issues during the QA/release process.

[emkll]

Should we open a follow-up task to completely eliminate trusty references at some point in the future, and/or replace the defaults with xenial?

[redshiftzero]

so I have task #4155 filed for this which this PR does not close (wanted to get this PR in first so that we can not trigger quite so many builds since we're getting a lot of queueing in circleCI)

[emkll]

LGTM !