Join pontoon_db.py and pontoon_non_db.py

[mathjazz]

We separate internal quality checks into two modules:

• pontoon_db.py: produces errors and warnings that are stored into the DB
• pontoon_non_db.py: produces front-end warning only

However, this is just a social contract. Placing checks into one of the modules does not mean failed checks will actually be stored into the DB or not. Additionally, we've broken that in https://github.com/mozilla/pontoon/commit/ba2435459f427c4bf72b688be776fd6afc28ef22.

Let's drop this separation completely and put all internal checks into one file.