refactor: `r/demo/users`

[leohhhn]

Description

Addresses: #2827

This PR refactors the current r/demo/users & r/sys/users system in accordance to the issue mentioned above. Here is the high-level overview:

• Removes the r/demo/users package in favor of r/gnoland/users
• r/gnoland/users becomes the main entry-point for users registering their usernames
• Modifies r/sys/users:
  • It now contains the address/name <> user store, so that the registration implementation (such as r/gnoland/users) can be agnostic and upgradeable, by providing a whitelist mechanism which allows write access to the store to certain addresses
• Implements better rendering
• Both r/sys/users & r/gnoland/users are ownable & pausable, currently by a single address, which could become a DAO at one point

WIP

Contributors' checklist...

- [ ] Added new tests, or not needed, or not feasible - [ ] Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory - [ ] Updated the official documentation or not needed - [ ] No breaking changes were made, or a `BREAKING CHANGE: xxx` message was included in the description - [ ] Added references to related issues and PRs - [ ] Provided any useful hints for running manual tests

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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