UI for password encrypted message functionality

[tomholub]

part of #955

[tomholub]

When there is least one recipient that doesn't have any pubkey, currently we don't allow the message to be sent.

For now, if domain of active user is not flowcrypt.com, keep current behavior (because this new functionality will be work-in-progress for a while, we don't want to allow it for external users yet).

Else, in situation when at least one recipient doesn't have pubkey available:

• add UI (on the top of the message under recipients?) with text: Tap to add password for recipients who don't have encryption set up.. Tapping could produce a modal like Set web portal password\n\nThe recipients will receive a link to read your message on a web portal, where they will need to enter this password.\n\n[input]\n[set] [cancel]. I'm not sure what is the best UI to use here. You can have a look at what browser extension does. After we set it, the text could change to Web portal password added. Tapping it again could maybe allow the user to set another one.
• when sending message, the password that was entered should be added into Core.composeEmail as pwd, along with the list of public keys
• if we send a message (to someone without pubkey) and the above was not set, you could produce the same modal that says Enter web portal password.... This time, after tapping ok, the message would get sent (using that password). Cancel would cancel sending, so the user could adjust recipients if they want.