Add permission required for email notifications

[asteflova]

What changes are you introducing?

The procedure to configure email notifications for a user is missing details on which permission is required.

Why are you introducing these changes? (Explanation, links to references, issues, etc.)

https://issues.redhat.com/browse/SAT-27647 and https://issues.redhat.com/browse/SAT-27616

With the exact permission missing, users might instead be tempted to grant a broader set of permissions (admin level) in an effort to troubleshoot the problem. We should try to avoid that.

Anything else to add? (Considerations, potential downsides, alternative solutions you have explored, etc.)

The error message displayed when the user account doesn't have the required permission is not very helpful:

I might look into expanding the error message next if I have the capacity for it.

Checklists

• [x] I am okay with my commits getting squashed when you merge this PR.
• [x] I am familiar with the contributing guidelines.

Please cherry-pick my commits into:

• [x] Foreman 3.12/Katello 4.14 (Satellite 6.16)
• [x] Foreman 3.11/Katello 4.13
• [x] Foreman 3.10/Katello 4.12
• [x] Foreman 3.9/Katello 4.11 (Satellite 6.15; orcharhino 6.8/6.9/6.10)
• [x] Foreman 3.8/Katello 4.10
• [x] Foreman 3.7/Katello 4.9 (Satellite 6.14)
• [x] Foreman 3.6/Katello 4.8
• [x] Foreman 3.5/Katello 4.7 (Satellite 6.13; orcharhino 6.6/6.7)
• We do not accept PRs for Foreman older than 3.5.

[github-actions[bot]]

The PR preview for 9d2274554e246dd377eed8504f25284a2b74bbed is available at theforeman-foreman-documentation-preview-pr-3258.surge.sh

The following output files are affected by this PR:

• Administering_Project/index-foreman-deb.html
• Administering_Project/index-foreman-el.html
• Administering_Project/index-katello.html
• Administering_Project/index-orcharhino.html
• Administering_Project/index-satellite.html

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[asteflova]

I'm adding tech review done based on the conversation in https://issues.redhat.com/browse/SAT-27647.

Style review is still needed.

[asteflova]

Merged to "master" and cherry-picked:

8eea964fab..8818dd1d2a 3.12 -> 3.12 c493be7af0..b37775ae04 3.11 -> 3.11 b81ad65c00..1f550b327b 3.10 -> 3.10 1cac39685d..ed024378c0 3.9 -> 3.9 58c89b3c64..b0070b580f 3.8 -> 3.8 db84943f20..74531f8af1 3.7 -> 3.7 cfd432d08c..30c115a8e7 3.6 -> 3.6 ad13123c07..3b39a875c7 3.5 -> 3.5