Allow selecting fee rate for channel opening

[shesek]

The fee rate field in the channel opening is currently an expert-mode-only feature and presented as a free-text field. Instead, it should display a select box with "slow", "normal" and "urgent", with the fee rate shown in parenthesis. When expert mode is enabled, the select box should be turned back to the free-text field we have today, to allow for manual entry.

Another thing to consider is using the more common sat/vB for manual feerate entry instead of c-lightning's sat/kB and sat/kW units.