[Webserver] Add E-mail notifications after crash

[blackjack2693]

Alex wants to be notified, if one container crashes. The detection is given by the exit code provided by issue #275, but it still needs some additions:

• Check periodically the status of the containers from the webserver.
• Send an e-mail to a specified address, if one container exits, especially if it exits with a non zero exit code.
• Create an observer for the email client
• Add a webpage to enter the E-mail address to the webserver