ui(import): Input custom rescan blockheight as year/month datepicker

In #621 the blockheight can be provided with a simple number input field.
According to the designs, it should be a year/month datepicker.


See if it is viable to calculate the blockheight form a date (minus some vagueness factor).
If it is, add the component to the "import wallet" flow, however, keep the ability to let a user still input the _actual_ blockheight the rescan process should be started from.

Also, as this is useful information, the creation date of the wallet should be displayed during the "create wallet" flow also.