Replace personalblock with .section

Lots of apps were using ```<fieldset class="personalblock">``` to get some default settings styling. We changed that to the simpler and more flexible ```.section``` class.

A normal settings section should now look like this:
```
<div class="section">
  <h2>Appname</h2>
  <!-- Content here -->
</div>
```

The ```section``` class and the usage of ```h2``` will ensure the correct styling, as defined in https://github.com/owncloud/core/commit/f105f1aac65564245f0ced1037bfeac121b91bd9#diff-d28ce211f72cb6409a66e6c8ab52d723R243

As a bonus (and one of the reasons I did this cleanup), this standard look can now also be used in apps outside of settings.

Please review @owncloud/designers. As soon as this is accepted, I will send a mail to the developer list so all app developers can change this too. (We should also ideally backport https://github.com/owncloud/core/commit/bf77c580c1e20400dbeb3bdbb7e3ddcd87880409 but that we will decide after this is merged.)