Review and make sure all 3rd party services are owned by active members

[btlogy]

The project does rely on 3rd party services and some might no longer be controlled by enough active members of the community (e.g.: Cachix is currently blocking CI).

This means those services can break PRs or prevent/delay a release w/o enough contributors having access to the relevant 3rd party provider to fix the problem (e.g.: expired token).

Let's review them all and maybe ensure at least 2 active members are in control.

Tahoe LAFS

Provider	Account	Owner(s)	Status
​GitHub.com	org: Tahoe-LAFS	brian, exarkun, meejah	maybe enough active maintainers?
​app.CircleCI.com	org: Tahoe-LAFS	same as Github	maybe enough active maintainers?
​app.Cachix.org	org: tahoe-lafs-opensource	exarkun	no active maintainers and blocking (see #4134)
​hub.Docker.com	org: tahoelafsci	unknown	maybe blocking #4137
​PyPi.org	project: tahoe-lafs	brian, exarkun, meejah	maybe enough active maintainers?
​test.PyPi.org	project: tahoe-lafs	meejah	only one active maintainers

In addition, the same problem can arise in some other projects related to Tahoe-LAFS:

ZFEC

Provider	Account/!Org.	Owner(s)	Status
​GitHub.com	org: Tahoe-LAFS	brian, exarkun, meejah	maybe enough active maintainers?
​app.CircleCI.com	org: Tahoe-LAFS	same as GitHub	maybe enough active maintainers but likely broken (same as #4098)
​app.Cachix.org	org: tahoe-lafs-opensource	exarkun	no active maintainers and likely broken (like #4134)
​PyPi.org	project: zfec	exarkun, meejah, sajith, vu3rdd, brian, zooko	maybe enough active maintainers?
​test.PyPi.org	project: zfec	exarkun, meejah, sajith	supposedly working

Magic Folder

Provider	Account/!Org.	Owner(s)	Status
​GitHub.com	Tahoe-LAFS	brian, exarkun, meejah	maybe enough active maintainers?
​PyPi.org	project: magic-folder	meejah	only one active maintainers
​test.PyPi.org	project: magic-folder	meejah	only one active maintainers

Tahoe Capabilities

Provider	Account/!Org.	Owner(s)	Status
​GitHub.com	Tahoe-LAFS	brian, exarkun, meejah	maybe enough active maintainers?
​app.CircleCI.com	Tahoe-LAFS	same as GitHub	maybe enough active maintainers?
​app.Cachix.org	org: tahoe-lafs-opensource	exarkun	no active maintainers and likely broken (like #4134)
​PyPi.org	project: tahoe-capabilities	exarkun, meejah	only one active maintainers
​test.PyPi.org	project: tahoe-capabilities	exarkun	no active maintainers

Alternatively, if one of those 3rd party providers or related projects is not required or for Tahoe-LAFS or obsolete, maybe we should properly archive them (there might be more).