Add advanced option to keep secret storage in memory for session

This adds a default-off option to keep the secret storage passphrase cached in
memory for the current session to avoid death by prompts.

src/components/views/settings/tabs/user/LabsUserSettingsTab.js

@@ -66,6 +66,7 @@ export default class LabsUserSettingsTab extends React.Component {
                     <SettingsFlag name={"showHiddenEventsInTimeline"} level={SettingLevel.DEVICE} />
                     <SettingsFlag name={"lowBandwidth"} level={SettingLevel.DEVICE} />
                     <SettingsFlag name={"sendReadReceipts"} level={SettingLevel.ACCOUNT} />
+                    <SettingsFlag name={"keepSecretStoragePassphraseForSession"} level={SettingLevel.DEVICE} />
                 </div>
             </div>
         );