Update usercap email to use standard email template

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What does this MR do and why?

This code update modifies the way emails are sent when a user reaches a certain usage limit. The subject of the email remains the same, but the email is now sent using a standard layout. Additionally, several tests have been added to ensure that the email meets certain criteria, such as not having Gmail Actions links and preventing users from unsubscribing through a footer link. These changes aim to improve the consistency and reliability of the email notifications sent to users.

Changelog: changed

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Ignore the missing logos. My gdk network config is screwy. This renders fine in a different env.

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Visit http://gdk.local/rails/mailers/notify/user_cap_reached