Isolate functions related to config in Config.php + add unit tests + code_sniffer.
options.php is not supported anymore, but its content will be automatically saved into config.php
Fixes #shaarli/Shaarli#41
*TODO*: update [documentation](https://github.com/shaarli/Shaarli/wiki#configuration).
Relates to #218
Removes "hidden" access to the following variables:
- $GLOBALS['config']['datastore']
- PHPPREFIX
- PHPSUFFIX
Signed-off-by: VirtualTam <virtualtam@flibidi.net>
I reviewed character escaping everywhere with the following ideas:
* use a single common function to escape user data: `escape` using `htmlspecialchars`.
* sanitize fields in `index.php` after reading them from datastore and before sending them to templates.
It means no escaping function in Twig templates.
2 reasons:
* it reduces risks of security issue for future user made templates
* more readable templates
* sanitize user configuration fields after loading them.
