Refactor reminder back-end to use a QB mapper

[ChristophWurst]

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Is your feature request related to a problem? Please describe.

The CalDAV reminder back-end uses the raw query builder to insert/update/delete rows, has it's own column casting logic and so on.

Describe the solution you'd like

This could be moved to a simple QB mapper that takes care of this out of the box. Plus, we gain some type safety in that service.

Describe alternatives you've considered

N/a

Additional context

N/a