Add UserInfo class and fetch quota via API instead of PropfindJob

Add UserInfo class and fetch quota via API instead of PropfindJob

The PropfindJob quota includes the size of shares and thus leads to confusion
in regard of the real space available, as shown in the UI.
This commit aims to streamline the behaviour with the Android and iOS apps,
which also utilize the API.

Details:
- Refactor the QuotaInfo class into UserInfo
- Use JsonApiJob (ocs/v1.php/cloud/user) instead of PropfindJob
- Let ConnectionValidator use the new UserInfo class to fetch
  the user and the avatar image (to avoid code duplication)
- Allow updating the avatar image upon AccountSettings visibility,
  using UserInfo's quota fetching

Signed-off-by: Michael Schuster <michael@schuster.ms>