Blob embeds respect show_diff_preview_in_email

What does this MR do and why?

Part of #300855 (closed). Closes #618244 (closed).

Evaluate the setting both for the embed context and target, i.e. turning it off for a given project disables that project's mailings having any embeds sent out, as well as that project's code being embedded elsewhere.

This is erring on the side of caution; you could argue it'd be enough to check the target only, but we can change it later if a customer voices desire.

We disable expanding blob embeds in Service Desk emails entirely. Again, can be changed later, but these go to external users and currently never have diffs/instance source code attached, so keep it that way.

How to set up and validate locally

1. Check this branch out and enable the blob_permalink_embed FF.
2. Include a blob permalink in a project comment to another user on your GDK. Here's my example:
   
3. Check letter_opener (mine's at http://gdk.test:3000/rails/letter_opener/). You should see something like this:
   
   Not the world's prettiest email; to be iterated on in a follow-up. :)
4. In the project you're working out of, go to Settings → General → Visibility, project features, permissions → Email notifications and uncheck the checkbox Include diff previews, then scroll down and click Save changes.
5. Repeat your comment.
6. Re-check letter_opener. You should see something like this:
   

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