simplify environment variables required.

instead of multiple keys/macaroons with different permissions we request only one.
if someone wants to use lnbits with an invoice macaroon they're free to do it and
we will just fail on 'pay' methods, as before.

this also grandfathers the previous environment variables names so everything keeps
working without people having to change their setups.

in the meantime some bugs with lntxbot and c-lightning were fixed and the `requests`
dependency was eliminated because I can't organize myself into meaningful chunks of
changes.

lnbits/extensions/example/views_api.py

@@ -3,7 +3,9 @@
 # add your dependencies here
 
 # import json
-# import requests
+# import httpx
+# (use httpx just like requests, except instead of response.ok there's only the
+#  response.is_error that is its inverse)
 
 from quart import jsonify
 from http import HTTPStatus