Add missing CLI endpoints for webUI, download and serve it from CLI web --web-ui parameter

[andrevmatos]

Description

LC CLI provides almost all endpoints from PC in a schema-compatible manner. Only a few config endpoints missing, which after implemented, should allow CLI to be a drop-in replacement for Raiden webUI, a nice user interface and admin panel for a raiden REST API-compatible client. Additionally, --web-ui/--no-web-ui command line options should control serving a pre-built version of the webUI in the respective endpoints.

This should give CLI users a better perspective of current node's state in a more ergonomic way than raw REST requests.

Acceptance criteria

• Users can use the webUI with the CLI
• There's an easy way to serve the webUI from a CLI node

Tasks

• Identify [possibly undocumented] endpoints needed by webUI
• Implement those endpoints with compatible schema
• Add build step/script to download and uncompress webUI
• Implement --web-ui to allow serving webUI files from /