Move Ihat request to initializer

Fail gracefully when the application starts by warning that
ENV['IHAT_URL'] has not been defined or the server cannot connect to the
specified server
In both cases document conversion is unavailable, AFAIK. @rizwanreza, please confirm. The error messages feel like they should be almost identical (document conversion unavailable), except for the first part (unable to connect, `IHAT_URL` not set).