Unify email layouts: devise emails

What does this MR do?

Unifies the appearance of all devise emails by applying the same layout to all of them.

The devise emails are updated to match the design of the pipeline emails.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Unifies the appearence of all devise emails to improve UX for the users.

Allows to customize all devise emails by setting a custom brand logo.

Follow-up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9049.

The development of this MR is sponsored by @siemens (/cc @bufferoverflow).

Screenshots (if relevant)

Emails from gitlab.com have the additional contribute header:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/24969

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/25570

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/25572