Default popular networks display enhancements

## Background
Both users and networks are getting confused because the network they want to add has already been added. We should make UI enhancements to show which networks have been added, and which ones are available to add.

## User Story
Within Settings > Networks, show all popular networks, and with the added networks grey them out with an "Added" label. With the available networks to add, provide the ability to add by applying an "Add" label next to the network name.

Further, we could show all available networks with two CTAs (Add / Switch) where Add is greyed out if the network is already added.

Nice to have: Instead of `Networks > Add a network` change to `Networks > Manage networks` where we group and segregate by curated networks (whitelisted by MM) and user-added networks in terms of order.

## References
- [Adding popular network acceptance criteria](https://consensys.slack.com/archives/G01PKA4QC3C/p1673485741157679)
- [Suggestions for managing popular networks list](https://consensys.slack.com/archives/CBHSSFEN5/p1673623530360219)

Start from the Settings > Networks UI and review the linked acceptance criteria and network-management suggestions. Define the network states and available Add/Switch actions, then verify that popular networks are shown, added networks are greyed out with an “Added” label, and available networks have an “Add” action.

Written by the indexing model from the issue text.