CreateIdentityWizard: Fix URI / nickname size limit handling:
- Use HTTPRequest.getPartAsStringThrowing() when checking validity to ensure that we get SizeLimitExceededException so we can tell the user that the string is too long
- Use HTTPRequest.getPartAsStringFailsafe() instead of deprecated getPartAsString() because the latter returns an empty string if the size limit is exceeded. The replacement returns as many characters of the original string as the limit allows
- Use proper size limit constant for nickname. Add TODO for FreenetURI size limit constant since there is none yet.