Misc improvements throughout the app

[jamaljsr]

Closes #45

This PR makes a bunch of small fixes and improvements throughout the app. These are all relatively small issues that have been accumulating over time.

• update swap loading icon colors to white & gray
• don't force channel selection when starting a loop. This allows users to perform a swap without needing to specify channels.
• fix navbar menu icon overlap with page header back arrow
• add text in Loop History tile when there are no past swaps
• prevent Loop In when multiple channels are selected using different peers. Also display a message informing the user why the "Loop In" button is disabled
• pass channel ids with Loop In/Out RPC requests
• fix findDOMNode console error (react-virtualized)
• set swap speed based on GetInfo 'chain' field. 30mins for mainnet/testnet, immediate for regtest
• refactor base styled components into a separate folder than common
• use big.js package to handle potential JS number overflow problems

[guggero]

LGTM, nice cleanup. Tested locally with the grub.