Add user cap to group's membership settings

What does this MR do and why?

It adds a user caps input to the group's settings under "Permissions, LFS, 2FA" -> "Membership".

Adding a user cap will give group owners control over accidental user overages.

Backend changes were picked from !72117 (closed), thanks @vij 🎉

Screenshots or screen recordings

before	after

How to set up and validate locally

1. rails console
2. my_group = Group.find(<group_id>)
3. Feature.enable(:saas_user_caps, my_group)
4. Visit the group's setting page on the browser
5. Check the new setting under "Permissions, LFS, 2FA" > "Membership"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #334296 (closed)