Get quota from SAML attributes

Sets user quotas from SAML attributes.

Adds two new settings:

* Quota mapping: an attribute that will be read to set the current quota
* Default quota: used to set a quota value when the quota attribute is missing.
  An empty default quota can be set to keep quotas untouched
Merge pull request #1880 from adoboPullRequests/saml_quota

user_saml: get quota from attributes