Make phone number in customer confirmation mail configurable and optional

**Motivation**
Currently there is always a phone number in the customer confirmation mail.
The phone number is not configurable but overridable using translations, which is not that convenient.
I also don't like that the shop kind of requires the shop owner to have a support phone number.

**Goal**
- Make the phone number easy configurable through the control panel
- Make the phone number optional for those who don't have a support phone

**Includes**
- Change for making phone number configurable and optional, including upgrade step for new registry setting
- New tests for customer and shop owner confirmation mails, using ftw.testbrowser
- Some additional markup in mail templates for easier testing
- Changes in tests for compatibility with newest ftw.testbrowser (`normalized_text` is now deprecated)
- Pyflakes cleanup

**Impact**
Sites which used to configure the phone number with translations need to update this to the registry configuration.
Other than that it should not impact any part of the shop.

@lukasgraf could you take a look at this change?