Introduce email templates for automatic background updates.

The four templates are:
 1. We successfully updated their site. If any of their themes or plugins are out of date, it also mentions that. ('success')
 2. We are not configured to update their site, so we notify them of the new release. ('notify')
 3. We tried but failed to update their site. The error was early in the process, so no harm, no foul. This is the same as template #2, plus one sentence. ('fail')
 4. We tried to update their site, and failed while copying files. ('critical')

With assistance from markjaquith.

see #10787.

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