Feature: adds recovery codes

[susgo]

Now it is ready for merge :-)

• save recovery codes (default 10) to new database table on enabling two-factor-authentication
• codes show up only one time to user (I have not tested with the given views because we overwrite them in our project)
• every code can used exactly one time
• php unit tests added and included via github actions

Please can you add a version tag for actual master and then after merging.

I look forward to your feedback. :-)