check for missing mnemonic

[n13]

Fix for #434

• On startup, check for mnemonic!
• If missing, log user out
• Setting to store mnemonic in encrypted backups (iOS)

Mnemonic still won't get stored in unencrypted backups, that's an iOS safety feature.

Testing

Change the code to always pretend like the mnemonic is missing, ie
instead of
if (mnemonic == null) {
do

  if (true) {

[dewabisma]

Overall looking good, just not sure that the change path about in launcher.yaml

[dewabisma]

LGTM!