v0.2.0

A broken root certificate is now reported and can be replaced, host setup
jobs survive a failed poll, and the plugin's internals are rebuilt as
layers with enforced complexity limits. Host requirements are unchanged
from 0.1.1.

• A root certificate that exists but cannot be used, such as one whose key
  is gone from the secret store, is reported instead of being read as "no
  root yet": Test reports root_unusable, hosts.add and hosts.update
  answer tls root_unusable for a host that terminates TLS with the
  plugin's certificate (not_proxy for one that does not), ca.get answers
  usable: false with the reason, and setup.status shows the root item as
  failing rather than failing as a whole. ca.create with replace now
  replaces it; without replace it answers 409. A failure to read the
  store stays an error to retry, and never replaces the root.
• A setup or upgrade job's view keeps following the job after a failed
  status poll, such as during a plugin restart, instead of staying on
  "Running" until the page is reloaded. A job the plugin no longer knows
  ends there, with the reason, and can be run again.
• The "add a host that is already set up" form no longer pins a host key
  scanned for one SSH destination after the destination was edited.
• A failure to read the plugin's SSH key, or the hosts' last check results,
  is no longer reported as the host registry's, and an admin login refused
  during setup is no longer logged as the plugin's key being refused.
• The maintainer no longer connects, or creates the plugin's SSH key, when
  no host is registered.