This occurs for attachments added via the staff or client web interfaces

Use .domain.tld to accept all subdomains of a parent

This is a slight edit over the previous implementation of the same feature.
Previously, an "account" was required, which implies a user account with a
password. This implementation simply requires a user record. Importing of
users by organization domain is still supported -- even if the user does not
yet exist.

  * Auto confirm remote accounts
  * Don't send out emails for remote account activation
  * Forbid password changes on remote accounts

This stems from a confusing similarity between the + operator for arrays
and array_merge() in php. Adding arrays will ignore items in the RHS where
keys are present in the LHS. Therefore, when adding numerically indexed
arrays together, only items on the RHS that have a key higher than the
greatest key on the LHS will be included.

Support adding remote user to an organization

It is the defacto standard to reference an inline image in an email by
referencing the Content-ID header of the referenced attachment in the @src
attribute of the image. For instance, `<imc src="cid:image001.png">` where
the `cid:` scheme of the attribute indicates that the referenced image URL
is a separate content of the email. The image attached to the email would
have an accompanying header like: `Content-Id: <image001.png>`. However,
some mail systems, including a certain fax to email service, do not
correctly indicate the location of the image with the content-id URL.
Instead, the referenced image tag would be `<img src="image001.png">`

This patch addresses the issue by searching the message body for all
references to attached content-id's in all @src attributes with or without
the content-id URL scheme indicator. Previously, such images would not be
displayed inline in the ticket thread.

References:
https://tools.ietf.org/html/rfc2392