[luci] Page for crontab configuration

[Steven Barth]

Hello forum2008,

> Please add a page for crontab configuration. Thanks.
We have added on with https://dev.leipzig.freifunk.net/trac/changeset/2861.
This is just a simple text-editor page with which you can edit the contents of 
the file /etc/crontabs/root directly.

A more advanced version will follow when UCI support for cron is available.

Greetings
Cyrus