feat: Add `Notes` column to reaction variations table

[JanCBrammer]

This PR addresses #1963.

For details about this feature have a look at the documentation over at https://www.chemotion.net/docs/eln/ui/details#adding-notes-to-variations.

TODO:

• [x] add mail merge field to Standard.docx (I can't get my hands on Microsoft Word atm): <<=v.notes>>. See https://github.com/ComPlat/sablon for details. See a0d1629966c09d869d0761abd62315f584a26549 (thanks to @FabianMauz!)

See #1987 for the DB migration.

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