Improve "enter profile code" phase of profile import modal

[anttimaki]

• Remove the somewhat unnecessary "You haven't entered a code" and "You may import the profile" badges that mostly just look awkward
• Show "Invalid code" badge only if a profile code has been entered
• Remove the big red button when a valid profile code hasn't been entered. This looked like an error state before user has entered the code. Just disabling the Continue button if the code is invalid seems sufficient
• Combine the two remaining buttons into one
• Prevent submitting the form when the download is already in progress, both via button and key press
• Move isProfileCodeValid helper to modal components as that's the only place where it's used. Remove the unnecessary argument to make the method a computed value