Add plugin architecture

[lukebp]

The long term goal for Politeia is that it becomes the industry standard for a web app built on the decentralized web tech stack and that it becomes a platform that people can use to spin up their own decentralized web apps. The decentralized web tech stack is built around decentralization, security, and privacy.

People should be to be able to use Politeia to run personal blogs, community forums, governance platforms, and any other type website that they want to run using the decentralized web stack.

To accomplish this, Politeia needs to use a composable, extensible plugin architecture where plugins can be built in external repos. The v1.0.0 set the foundation for this architecture on the backend. This same type of architecture now needs to be added to politeiagui.

From a high level, the politeiagui architecture and UX is going to be similar to the architecture that Atom uses. Atom is a text editor that Github built. It is written entirely in JavaScript and markets itself as "A hackable text editor for the 21st Century" because of the composable, extensible plugin architecture that it uses. Users can customize their Atom editor using a built in package (i.e. plugin) manager or they can write their own packages. These packages are also written in JavaScript.

This is the direction that we want to head in.