[topsites] Refactor the code to make it more modular

[karlcow]

To make the code slightly more readable in the future, we could probably gain by making it more modular. Previous issue about it was #2015

• break down with more functions
• separate the strings in a file as string templates.
• adds more tests

See https://github.com/webcompat/webcompat.com/blob/2fb2e27253ae86d00a2d304395e39246fc0d923f/tools/topsites.py
and https://github.com/webcompat/webcompat.com/blob/2fb2e27253ae86d00a2d304395e39246fc0d923f/tests/unit/test_topsites.py

[karlcow]

When doing that. The current code takes 6 minutes to run (aka create a new db).
There might be optimization refactoring to do.
Why it's not really an issue for now. We run it only once a month… so 6 minutes for once a month is not necessary an issue to solve.