Make storing of the "original" start and end dates optional

Problem/Motivation

Currently we store the originally entered start and end range, but we do not actually use that at runtime.

This has two reasons:

1. It enhances interoperability, i.e. it allows easily migrating back to a regular date-time-range field and only lose the "recurring" feature
2. It increases debuggability, i.e. if something with the scheduling is not as expected it is easier to deduce the user's intent with this

However, it makes sense to allow people to avoid the extra storage if they do not want it.

Proposed resolution

Add a field storage setting to disable the value and end_value properties.

Data model changes

Optionally less data cruft!