Feature: enable/disable permalinks for RSS

The option to see the shortlinks or permalinks has been added to the configuration panel. It is a simple checkbox
This option is disabled by default (meaning that shortlinks are the default)
Updated writeConfig() to save this option
Also fixed a slight typo in config.html.
Reminder to update the configuration section of the docs (https://github.com/shaarli/Shaarli/wiki#configuration) when this gets merged.