Remove all format_* wrappers

[TkTech]

Our API is never up to date with babel (ex: #142), and we do not cover the entire babel API. Our wrappers add no real value, and add a lot of surface area for test coverage. Just like flask-wtforms did way back in version 0.9, we should focus on just being the glue between flask and babel.

• [x] format_datetime
• [x] format_date
• [x] format_time
• [x] format_timedelta
• [x] format_number
• [x] format_decimal
• [x] format_currency
• [x] format_percent
• [x] format_scientific

As part of this, we need to improve the documentation to show better examples of using get_locale and get_timezone.

[homeworkprod]

Hi @TkTech,

while I do agree with this move, I'd like to point out that while Flask-Babel's format_date rebases according to the time zone, Babel's format_date does not. This might be a bad surprise for users after upgrading to v3.0.

I have raised an issue against Babel, hoping that Babel will gain this ability. But I guess it could use some more backing.