Add option to assign quota to guest users by default

The problem:

Guest users are by default severely limited when it comes to using Nextcloud and being able to interact with other users efficiently. Currently, unless an admin steps in and assigns them quota, they cannot do the most basic tasks such as uploading and sharing files with other people, creating documents, posting files to talk conversations, and more.
The process of assigning quota to them is manual, tedious, and requires coordination between two or more people.

Current common workarounds:

• "Manually" asking any non admin user to share a folder with them where they are granted more permission. This requires one interaction with a person. And only partially solves the problem.
• "Manually" asking an admin user to step in and assigns some quota to them. This requires at least one interaction with one person, but possibly more since it's harder to reach admins.

Possible solution # 1

Allow admins to set a guest user policy. For example:

• Default guest quota (e.g. 1gb)
• Max global quota for guests (e.g. 50gb)
• Max guest users (with quota) per user

Pros:

• Achieves the goal
• No hassle for non-admin users

Cons:

• Complex settings for admin users
• Requires complex UI to keep track of everything

Possible solution # 2

Allow any user to hand over a portion of their quota to a guest users upon creation. We saw in current workarounds that this is effectively what's already happening, so this would just provide a more appropriate UI for it and reduce the amount of interaction between people to achieve the goal.

Pros:

• Achieves the goal
• Doesn't add complexity to both non-admins and admin users

Cons:

• Can be prone to abuse.

Possible solution # 3

A combination of the previous two where users can hand over quota to guests but within certain limits.

Pros:

• Achieves the goal
• No hassle for non-admins

Cons:

• some complexity introduced for admins

cc @nextcloud/designers for further discussion/input
Also cc @AndyScherzinger and @skjnldsv since we talked about it in #1316

The issue names no files, tests, or entry points. Read the three proposed quota-policy approaches and clarify whether the intended scope is admin settings, user-controlled quota sharing, or both; completion criteria are not yet defined.