Extract logic from run-report script into an action module

Summary:
To allow the MediaWiki client call the same logic than the run-report
script, we need three things for generate_report:

1. to be in a specific module callable from Python code
2. to move error handling from the generate code to the caller
3. to output both the result of connector and format callbacks

The Report and RawReport classes will allow the bot to perform analysis
on the report content to know if it's worthwhile or not to update the page.

Ref T2124

Test Plan:
Refactored with real-time control of report output:
gwatch "bin/run-report agora-operations-grimoire-older-pages"

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T2124

Differential Revision: https://devcentral.nasqueron.org/D3677