Allow Jira to embed images in email messages

Currently, when an email containing an image is sent, the email message contains a direct link to the image.

This can create problems if:

• the configured mail server uses proxies to cache images, is unable to authorize to Jira, and caches the 403 responses (e.g. when Gmail with the default settings is used as the outgoing mail server)
• the user is not logged in when viewing the email message, getting authorization error when querying for the image
• a mailing client is used that doesn't allow to set up a session inside Jira for authentication purposes

As a solution, Jira could give admins an option to embed the images inside emails.
Such a solution needs to be toggleable, as it adds a potentially expensive step to email creation, e.g. if a user attached multiple big images in one comment.