Add pre-receive hook to check for email privacy

[jolheiser]

This PR aims to help keep email private for those users who wish to do so.

It adds a check in pre-receive that checks incoming commits and checks if the commit author email matches the user committing.
If it matches any email and the user has indicated they want their email to remain private, the hook fails, telling them why.

This PR also adds a message to the user profile where they set their email privacy, letting them know what their noreply alternative is.

Now, a few questions and why this is a draft...

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1. Wording. I'm not a huge fan of the wording I've chosen, and I would like feedback.
2. Should a push option be added so a user can force the commit through, rather than making them open the web UI to change their privacy?
3. Currently this only checks commit author. Should it also check committer?

[zeripath]

Moving to 1.16 as not ready. Feel free to move back to 1.15 if ready.

[yardenshoham]

This is in progress for a while now, I'm closing it to avoid stale pull requests. Please reopen when it's ready for review.