i2p: Improve the application

Basic functionality for the I2P application is merged in !1472 (closed). However, since the proxies are not working it was not enabled yet. The following are the list of improvements sought on the app.

• Configure HTTP, HTTPS, SOCKS proxies on 0.0.0.0 and ::0 by default. Open these ports using firewall for internal zone. Use JSON-RPC to communicate with the I2P server if necessary.
• Write manual page for I2P at https://wiki.debian.org/FreedomBox/Manual/I2P
• Document using Tor Browser with I2P SOCKS proxy in the manual page.
• Add functional tests for checking basic UI and backup/restore functionality.
• Ensure that existing favourites sites provided by upstream I2P are not removed when our sites are added.
• Add diagnostics tests for HTTP, HTTPS and SOCKS proxy ports too.
• Add separate front page shortcuts for tunnel configuration, torrents and proxies.
• Add Tor browser to clients list.
• (optional) Explore if use of proxy_html can be eliminated by configuring the path /i2p/ in I2P daemon.
• (optional) Use augeus to edit the router.config for adding favourites. Or even better, communicate with the daemon using JSON-RPC to update the configuration file. This might also solve problem with existing favourites getting removed.
• (optional) Explore if any firewall ports need to opened up to make participation in the peer-to-peer network easier.
• (optional) Explore exposing FreedomBox services over an eepsite.
• (optional) Explore other I2P services that could be exposed to users.