add readonly mode

[robkooper]

tested with create spaces/datasets/files but might be missing special cases. fixes #405

Description

A request was made to be able to mark users as read-only. They should not be able to create new spaces, or anything else. All they can do is view things.

Use 2 users, an admin user and set the second user to readonly

See if the readonly user can do any write operations, create spaces/datasets/collections/files/tags/comments etc.

Review Time Estimate

• [ ] Immediately
• [ ] Within one week
• [X] When possible

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [X] I have updated the CHANGELOG.md.
• [X] I have signed the CLA
• [ ] I have updated the documentation accordingly.
• [X] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[max-zilla]

tested this, was able to create a dataset using Create Dataset button on the explore page when logged in as user with status set to ReadOnly (db.social.users.update({ "_id" : ObjectId("64248e486635dbcac3f867ab")}, {$set: {"status": "ReadOnly"}});)