Enhancement: Option to allow quotas set in NC to override the SAML quota

Was having issues with both this and roles. I saw PR #311, and was wondering if we could have something here as well that respected the changes done on the nextcloud side.

Steps to reproduce

1. Start with base, functioning nextcloud + saml, with a mapped quota attribute
2. Set quota on saml user, log in with that user, and verify that that quota takes effect
3. As admin, manually change that users quota
4. Log in with the saml user, and see that the quota has been reset.

Expected behaviour

The quota should stick to the new value set via nextcloud interface.
Should possibly be an option to use the quota attribute as a default value.

Actual behaviour

The quota stays at the value defined in saml.

Server configuration

Keycloak + Nextcloud 15.0.7 + PHP 7.2

Operating system: Ubuntu 18.10

Web server: Nginx

Database: MariaDB

PHP version: 7.2

Nextcloud version: 15.0.7

Where did you install Nextcloud from: Tarball

Start by reviewing PR #311 and the existing SAML quota-mapping behavior described in the reproduction steps. Trace where the mapped quota is applied during login and determine how a Nextcloud-set quota could take precedence. Done means an option or behavior allows the manually changed Nextcloud quota to persist while retaining a configurable SAML quota default.