Answer to a notification by email

When a notification from a mention which a user can answer (e.g., mentions and notifications) is received by email, it should be possible to answer the email directly.

The answer is then integrated as an answer in the web interface.

Answering by email follows the same logic as UC8. Answering to a notification from the specifications.

See https://design.xwiki.org/xwiki/bin/view/Proposal/UserMentions#HUC9.Answertoanotificationbyemail