Support kraken staking assets with set unstaking days

[LefterisJP]

Problem Definition

A user came in our discord and complained they get weird assets such as SOl03. GRT28. KAVA21. ATOM21. DOT28. in rotki.

We have code that reads the staked assets and maps them to the actual asset here:

rotki/rotkehlchen/assets/converters.py

Lines 894 to 901 in e91be08

	if kraken_name.endswith(('.S', '.M')):
	# this is a staked coin. For now since we don't show staked coins
	# consider it as the normal version. In the future we may perhaps
	# differentiate between them in the balances https://github.com/rotki/rotki/issues/569
	kraken_name = kraken_name[:-2]
	if kraken_name.endswith('.HOLD'):
	kraken_name = kraken_name[:-5]

Kraken later added options for different staking APY by adding days to unstake for the asset. More days to unstake, more APY.

But it seems that by doing so kraken also added new undocumented functionality to the API and now the staked assets may also contain the number of days between the asset symbol and the .S. So GRT28.S, ATOM21.S etc.

Task

Fix it. Probably by making the asset_from_kraken function take into account potential numbers in between and ignore them.

[LefterisJP]

This should now be fixed and either go to 1.30.2 or 1.31.0