Adding read-only & disabled mode for Logplex API

The Disabled mode entirely shuts down access to the web API at all
levels, including for healthchecks. Use with extreme care.

The read-only mode forbids all non-GET operations outside of what v2
canaries are allowed to do.
Merge pull request #61 from heroku/disable-api

Adding read-only & disabled mode for Logplex API