This allows the directory where CSS/JS asset collections are
written to be changed, in case SERVERROOT is not writeable. Note
it does *not* allow the expected URL to be changed: whatever
directory is used, the server must be configured to serve it
at WEBROOT/assets. It may be possible to add another config
parameter to allow the admin to specify a custom asset URL,
but I thought I'd keep the first implementation simple.

At some point SeparatorFilter should be included upstream
(kriswallsmith/assetic), then lib/private/assetic/separatorfilter.php can be
removed and the `use` in lib/private/templatelayout.php rewritten.

SeparatorFilter inserts a separator between assets, preventing issues when
files are incorrectly terminated. For JS this is a semicolon.

* fix #7484
* use UID, css, and div instead of span

There is no need to refer to the absolute path here if we can use the relative one.

Conflicts:
	lib/private/templatelayout.php

adding argument deleteSelf to rmdirr() - if false the directory itself will not be deleted only it's content

adding repair step to clean the asset cache after upgrade + coding style adjustments

This leads to the regeneration of the hash in case a single application is updated.

Fixes https://github.com/owncloud/core/issues/11374

Conflicts:
	core/templates/layout.guest.php

Step one, open guest layout for different styles.

Conflicts:
	lib/private/vcategories.php

And other small improvements

 - Use camelCase

 - Add some phpdoc

 - Fix some indents

 - Use some more spacing

revert changes to fbbc76f2 because master is very unstable right now