[LND] Add advanced channel policy min/max_htlc_msat

[fvdpol]

This PR extends the 'Update Fee Policy' for a specific channel with an advanced options section that exposes the minimum_htlc_msat and maximum_htlc_msat channel policy parameters.

From a user's perspective the change is minimal, as the additional parameters are not exposed and only visual difference is the presence of a 'Show Advanced' button at the bottom of the form:

After clicking the 'Show Advanced' button the layout is changed and the additional fields exposed:

Tested with LND 0.14.2