Keep BTC price updated

The NodeJS script at cronjobs/fetch_btc_rates.js can be used to fetch rates from a public API at a predefined interval and put them into the database.

The script depends on the following .env parameters:

API_KEY_COINAPI - API key from CoinAPI, the currently used conversion rate service

CURRENCIES - comma separated list of currenceis for which rates are to be retrieved

RATES_DOC - document name in CouchDB where the rates are saved

COUCH - IP address of CouchDB instance

COUCH_PASS - admin authentication token

Sample .env section:

API_KEY_COINAPI=DE914320D-09EB-4DS7-8D1A-219A8ESDF052
CURRENCIES=EUR,USD,GBP
COUCH=http://localhost:5984
COUCH_PASS=bW9uZXlwcmludGVyOmdvYnJycg==
RATES_DOC=rate:10kSATS