Config sample cleanup

With this PR I want to enhance the quality of the documentation.

The idea was to generate the "What config options are available and what do they do" documentation page from the `config.sample.php`, because it's hard to keep both in sync. @carlaschroder did a great job at the documentation part and we should work together as this means less repeating work for ourselves. We also push the developers to document new config options in the config.sample.php, so why don't we reuse that effort and generate documentation right from this.

I wrote a little script that parses the config.sample.php and generates a RST file. https://github.com/MorrisJobke/ownCloud-config-converter

Output: https://gist.github.com/MorrisJobke/31aad4fff99d97b0fc1c

You maybe think: "OMG! This is a huge PR, which changes nearly everything!" Yes. But I splitted it into some commits. The first introduces proper comments (starting with `/**` and ending with `*/`) which has no more than 80 characters per line. The second one moves everything around and I was inspired (I used exactly the same order) by @carlaschroder 's PR (owncloud/documentation#594). The last one ist just a cleanup of all of this stuff.

What do you think? Is this a good idea? Could this work? I also can try to write a test case for a valid (parse-able by my script) config.sample.php?

cc @karlitschek @craigpg @DeepDiver1975 @danimo @wakeup 

=> "Specifies whether avatars should be enabled"