Fee rate has changed - notify user in transaction modal

[mroz22]

Problem

1. user enters custom fee rate into send form
2. with given feerate an output lower than dust would need to be created so this output is omitted and actual fee rate is different
3. user still sees the same fee rate in transaction overview modal

Proposed solution if we detect that the actual fee rate is different we should:

• do not change fee rate in user input
• do change fee rate shown in transaction overview modal
• show tooltip in transaction overview modal with an explanation why fee rate is different