Add memo encryption / decryption capability

[stoodkev]

Is your feature request related to a problem? Please describe.
Currently there is no way to decrypt or encrypt memos.

Describe the solution you'd like
Decryption from transfer history and encryption from transfer modal.

Additional context
This task is harder than expected and I already spent quite some time on this. The reason for this is that the functions used for encryption in @hiveio/hive-js use the core node module crypto that is not available in react-native context. I have tried some known workarounds so far, not to avail (in production).