feat(external data): loading currencies

* Added an app in the backend dedicated to the link with external data
* Create a command to update the data
* Support updating data from fixer
* Updated doc accordingly

Almost done #28
Status Job ID Name Coverage
  Check
passed #554068
docker
check_back

00:00:18

passed #554069
docker
check_front

00:01:02

 
  Test
passed #554070
docker
test_back

00:00:52

91.0%
passed #554071
docker
test_frontend

00:00:18

 
  Lint
passed #554073
docker
eslint

00:00:22

passed #554072
docker
flake8

00:00:13

 
  Svg Gen Docu
passed #554074
docker
generate_UML_svg

00:00:12

 
  Documentation
passed #554075
docker
pages

00:00:12

 
  Deploy
passed pages:deploy

00:00:00