* add new settings in Accouts Settings
* patch for new settings
* refactor `get_exchange_rate`
* adds validation
* tests validation
* disables conversion rate field if stale rates not allowed
* more test cases
more test case...
test `get_exchange_rate` behaviour with stale not allowed in sett..
fix currency exchange test case
do housekeeping after running accounts settings test
* clean up
* documentation
* make use of correct api url
* Fix tests failing due to wrong exchange rate from fixer.io
* remove mandatory constraint from `allow_stale`
* added info to documentation
