Added "Reminder" to subject line of follow up asset checkout emails

[marcusmoore]

This PR updates the subject line for emails sent via the "Reports: Unaccepted Assets page" (/reports/unaccepted_assets):

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Since the same Mailable, CheckoutAssetMail, is used when the initial email is sent and when the buttons above are clicked receivers of the email may think additional assets have been checked out to them.

This differentiates the initial and follow up emails by prefixing Reminder: to the subject line:

Situation	Result
Initial Email
Additional Emails

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My implementation included adding another parameter, $firstTimeSending = true, to an already long parameter list and I don't love that but it works.

I also added tests for different logic branches in the controller method.

[what-the-diff]

PR Summary

• Improved Email Notifications For Reports
  The email logic within the reports section has been adjusted to send reminder notifications more effectively. Emails are now tailored based on it being the first alert or a subsequent reminder.

• Enhancements To The Asset Checkout System
  A key component that controls asset checkout has been improved to differentiate between a first alert and a reminder. This enhancement results in more accurate and efficient notifications.

• Updated Email Language
  New language keys have been added to the email system to accommodate the improved reminder system. This enables the creation of more fitting email subjects when reminders are sent.

• Inclusion Of New State In The Checkout System
  The checkout state now has an additional, 'accepted' status which denotes a completed checkout procedure. This enhancement improves the system's ability to accurately track and record checkout statuses.

• Added Unit Tests For Reminder System
  The new reminder system now includes dedicated unit tests. These tests ensure the proper functioning of the reminder system and identify any unexpected behaviour efficiently.

• Refined Existing Tests
  Minor changes have been made to existing tests to align them with the updated mail logic, ensuring consistent evaluation of the email notification system.