Add web bot querying services for updates in provider ratings

This MR adds a WebBot, which queries services for updates in provider ratings. Currently implemented services:

• compliance.conversations.im (part of #32)
• www.thegreenwebfoundation.org (part of #71 (closed)) (only query for now)

Changes are written directly to providers.json, which can then be used for an automatic MR:

python -m automation.web

To trigger an update on compliance.conversations.im, run WebBot with -u or --update:

python -m automation.web --update