Add ability to email users from the map

[tomnixon]

Just a stub for now - needs UX design.

User requirement: An easy way to send a message to another organisation member from within Maptio. Does not require a bespoke messaging system. It could simply be a mail icon that appears next to somebody on their profile page on the directory which would open a form that would send the other user an email, with a checkbox to also copy the message to the sender. Later, this feature could integrate with other messaging systems such as Slack.

(Requested by Deborah Chang from Youth Power Coalition)