Support negative inbound fees

**Is your feature request related to a problem? Please describe.**
Negative Inbound fees were introduced in `lnd v0.18.x` 

**Describe the solution you'd like**
Support inbound fees in GUI where relevant. 

**Describe alternatives you've considered**
n/a

**Provide examples**
n/a

**Additional context**
https://github.com/lightningnetwork/lnd/pull/6703
