Html and text files are not fixed to be UTF-8. The encoding is now
detected, which may not work for all files. Default/fallback will be
utf-8.
There is still a problem with mails that contain html parts not in
utf8 encoding. The mail text is always returned as a string and the
original encoding is lost. Then the html is stored using utf-8 bytes,
but wkhtmltopdf reads it using latin1. It seems that the `--encoding`
setting doesn't override encoding provided by the document.
Periodic tasks are special in that they are usually kept around and
started based on a schedule. A new component checks periodic tasks and
submits them in the queue once they are due.
In order to avoid duplicate periodic jobs, the tracker of a job is
used to store the periodic job id. Each time a periodic task is due,
it is first checked if there is a job running (or queued) for this
task.
This client can be used within the backend app and later in other
modules. The `OJoex` object is replaced with a better implementation
where the http client is initialized once on app start.
