feat: added UI for handling namespace maintainers

[arteevraina]

• [x] Fixed the logic for fetching namespace & packages of a user.
• [x] Hide Remove Maintainer from package option if user is not a namespace admin/maintainer.
• [x] Added UI for adding / removing namespace maintainer.
• [x] Fixed the API for adding/removing package maintainers to allow namespace maintainers to add/remove package maintainers.
• [x] Hide Remove Namespace maintainer button if user is not a namespace admin.

[arteevraina]

It can be tested here - https://registry-frontend.vercel.app/

[minhqdao]

The website seems much more fluent and lots of the points that were mentioned in the past are resolved – thank you!

I might be mistaken, but the website also seems to respond faster than last time.

It's also great that we can add and remove maintainers to and from packages as well as namespaces.

Some notes on upcoming features: I'd still prefer decreasing versions here: (the latest version should be up because it's the most relevant)

Also clicking the version doesn't bring me to that specific version.

We should also not forget that there was the request to allow an admin to add other admins that have the same rights as the original admin.