Make Xenial development/CI environments the default

[redshiftzero]

Description

New installs beginning with 0.12.0 will be done on Xenial. Xenial will become the primary base OS for SecureDrop, and we will deprecate support for Trusty when it EOLs in April. We should thus:

• Make the default CI staging job that is run per-PR xenial ([ci] move trusty app tests to nightly job and use circle ci branch filtering logic #4158)
• Make the trusty CI staging job run nightly ([ci] move trusty app tests to nightly job and use circle ci branch filtering logic #4158)
• Rename the makefile targets that are staging-xenial and test-xenial just staging and test (renaming the existing staging to staging-trusty so that we keep them around to ensure we don't break anything for installs that have yet to migrate) (Rename Makefile targets, Xenial new default #4224)
• Make BASE_OS default be xenial instead of trusty in the securedrop application Makefile ([dev] some updates for making Xenial default env #4213)
• Update and test Qubes staging setup docs on Xenial base OS (dev only)
• Update Vagrantfile to use Xenial instead of Trusty (probably only used for Prod VMs at this point) ([dev] some updates for making Xenial default env #4213)
• Port "upgrade" scenario to Xenial, so VM upgrade testing can be performed on Xenial by default (Port upgrade scenario to Xenial #4237)
• Update default staging target to Xenial (Update default staging target to Xenial #4310)

[redshiftzero]

everything here was done, anything we forgot should get a new dedicated issue