Rename pi-ui

[xaur]

pi-ui is no longer specific to Politeia since it is also actively used by Decrediton. How about updating repo/module name to reflect its current function? I think it can be summarized as "a collection of UI elements for building Decred apps in React" so it is like a specialized widget toolkit. If this is correct, the name could be like decred-react-sdk (copying the style from matrix-react-sdk).