option to keep pass phrase only in memory #197
Description
part of #275
During setup (of new key or when importing key or when loading from backup - whenever I'm creating or entering pass phrase), the user should see two radio buttons:
o store pass phrase locally
o keep pass phrase in memory
Default is to store.
If the user switches it, then we do not store pass phrase with the key (or at all). We only keep it in memory for up to 4 hours from the moment it was stored - then it needs to be forgotten. During those 4 hours, the key will be used for actions (eg decrypt messages). After those 4 hours, the user will be prompted for a pass phrase with a modal / alert to re-enter it, at which point it will be again remembered for 4 hours.
If app gets killed, pass phrase gets forgotten.