Allow for multiple training partners

[Zach]

Instead of storing training partners as a single string, store similar to tags or equipment. This would allow for searching for multiple training partners and may simplify picking the same partners consistently (eliminating spelling errors, typos, etc.)

[mipapo]

Nice to have, but very unimportant in our point of view

Added as an idea in https://github.com/Runalyze/Runalyze/issues/1473