Turn formality into a checkout setting

@JurriaanRoelofs
Some time ago, there was a similar issue concerning the formality level per language. But the suggestion to add the selection right to the checkout process makes more sense.
Please test the MR and give feedback in this issue.

Testing instructions

• run database udpates to get default values for new config setting override_formality
• ensure, that new checkbox Allow override of Formality in checkout is available in configuration form of the DeepL translator
• if Allow override of Formality in checkout is enabled, you'll be able to set the formality level while checkout
• ensure, that the override has an effect on the translations

@JurriaanRoelofs
Did you find time testing the Merge Request?
Besides asking for new features, it would be great, if you could help testing issues ;)..