Split footer css into its own file

[Randy808]

Part of #665

To run locally, npm install and use the following snippets:

# Bitcoin
export API_URL=https://blockstream.info/api
export PORT=4999
source flavors/blockstream/config.env
source flavors/bitcoin-mainnet/config.env
npm run dev-server

# Liquid
export API_URL=https://blockstream.info/liquid/api
export PORT=5000
source flavors/blockstream/config.env
source flavors/liquid-mainnet/config.env
npm run dev-server

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There should be no difference between the 2 footers aside from the border color change on the social tiles (a bug fix). Everything else is purely a refactor.

Before:

After:

.