Migrate new app settings to AppFramework

Let's migrate those two new files.
Generate stable etags for local files
allow passing driver options, fixes #11718
add driver options to config samples
Merge pull request #11763 from owncloud/stable-etags

Generate stable etags for local files
Merge pull request #11783 from owncloud/allow_passing_driver_options

allow passing db driver options
Properly catch 503 storage not available in getQuotaInfo

When doing a PROPFIND on the root and one of the mount points is not
available, the returned quota attributes will now be zero.

This fix prevents the expected exception to make the whole call fail.
On Windows platform we have no stable etag generation - yet
Make supported DBs configurable within config.php

This commit will make the supported DBs for installation configurable within config.php. By default the following databases are tested: "sqlite", "mysql", "pgsql". The reason behind this is that there might be instances where we want to prevent SQLite to be used by mistake.

To test this play around with the new configuration parameter "supportedDatabases".
code cleanup during review :+1:
Merge pull request #11786 from owncloud/MakeSupportedDBsConfigurable

Make supported DBs configurable within config.php
Merge pull request #11799 from owncloud/no-stable-etags-on-windows

On Windows platform we have no stable etag generation - yet
[tx-robot] updated from transifex
Merge pull request #11791 from owncloud/webdav-getquotainfo-503-handling

Properly catch 503 storage not available in getQuotaInfo
Merge pull request #11600 from owncloud/refactor-appsettings-to-app-framework

Migrate new app settings to AppFramework
use Composer autoloader not OC for non-Composer 3rdparty (#9643)

Composer's autoloader is rather better than the OwnCloud autoloader's
handling of non-OC classes. Plus we can rely on upstream Composer to
maintain it and not worry about it ourselves.

With this change, we drop the bits of OwnCloud's autoloader that
handled non-OC classes, and register the classes that were being
handled by that code with Composer's autoloader instead. As these
dependencies are converted to actually being managed by Composer,
the explicit registrations can be dropped as they won't be needed
any more.

Since OwnCloud's autoloader isn't going to handle non-OC classes any
more, we no longer need to test to make sure it does it right.

drop unneeded registerPrefix() and registerClass() from autoloader

Now we're not handling anything but OC's own classes, these are
unnecessary.

error out if composer autoloader is not found (thanks bantu)

We're never going to be able to work without the autoloader, if it's not
there we should just throw our hands up and surrender.
Merge pull request #11805 from owncloud/AdamWill-issue9643

use Composer autoloader not OC for non-Composer 3rdparty (#9643)
Use composer autoloader to load Patchwork