Make fingerprint verification more easy

[link2xt]

@Max1

Show a “secure” icon on the send button once e2e encryption is active

There is an existing feature proposal at

show end-to-end encryption state of chat

when you want to be sure end-to-end crypto is used, you have to manually check if you reply to a message with a lock symbol. else your message will get sent unencrypted. there should be a notification, when end-to-end crypto is downgraded to disabled. also a force encryption option may be useful: #246. similar to #212. (This proposal has been moved from the issue tracker)