Feature/fee slider

[malteish]

See issue #328

Proposal: Enforce fee limits and compare fee amounts on per-product base instead of comparing fee types directly.
Pro:

• simple
• allows for fee slider in frontend
  Con:
• complications especially with employee participation, since many fees for EP are handled off-chain. (E.g.: founder decides for 0 token fees. How do we change our cash fees to account for that?)

[openzeppelin-code]

Feature/fee slider

Generated at commit: 98f38de116bbe7640eb550a99709bba49724faff

🚨 Report Summary

	Severity Level	Results
Contracts	Critical High Medium Low Note Total	0 0 0 8 35 43
Dependencies	Critical High Medium Low Note Total	0 0 0 2 44 46

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For more details view the full report in OpenZeppelin Code Inspector

[CJentzsch]

Looks good.

For EP, we have to adapt the off-chain fees when lowering the token fee. But from a smart contract perspective, this is good.