Add "email" link to sharing options

[e-t-k]

Current "sharing" dialog should have am "email" option added.
Would open the user's chosen mail client and generate an email with any images in the post embedded as attachments.

[e-t-k]

Commit c629098 in reference to this issue; adds simple mailto: link. Necessary further improvements:

• Improve appearance of sharing dialog -- make email icon inline with other icons
• make email icon same size as other icons

Possible further improvements needing discussion as to whether and how to implement:

• If a post includes a link, all sharing options link directly to that link instead of to the post. Would suggest linking to the post instead.
• use Telescope.email.buildAndSend to send email directly from MedBook. This (should or should not?) include post contents and any files attached to the post
• Collaboration awareness: warn (or forbid?) the user if the post is not otherwise publicly visible.