[UserCap BE]: Add User Cap setting

Why are we doing this work

In &4315 (closed), we introduced a User Cap setting for self-managed admins to prevent accidental user overages, now we are doing the same for SaaS group owners.

This work will be broken down into the following backend steps:

• Add user cap setting 👈 we are here
• Adding/inviting members
• Notifications
• Member management
• Auto approval
• Approval Modal
• Group access modifications
• Group sharing
• UserCap availability

This issue is to handle adding the ability to assign a user cap value to a group. This will be managed within a Group's settings.

Relevant links

• Epic: &5803 (closed)
• Designs: #321934[V4.png]

Implementation plan

• Add a new column to store user cap number to the Namespaces table
• Add ApplicationSetting support for new setting
• Add ability to update the user cap setting for a root level group
• Calculate and expose user cap status (cap reached etc) for front-end use (see: #330028 (closed))
• Introduce a feature flag for controlling access to User Cap functionality, e.g. exposing the status in the point above

Edit: I've removed the second point above because I'm not sure it's actually needed - this is a group/namespace level setting, not an instance-wide one