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

[theborakompanioni]

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.

[theborakompanioni]

Blocked by #621