Forlæng invitationer

[rasmusselsmark]

Løser #1074 på http://localhost:8000/admin/members/activityinvite/

Nyt menupunkt til at forlænge invitationer:

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[what-the-diff]

PR Summary

• Introduction of New Functionality to Admin Interface
  The changes here involve importing several additional libraries to support new functionality. The primary change is the addition of a method extend_invitations to the ActivityInviteAdmin class. This gives administrators an improved way to work with invitations.

• New Form for Admin
  A new class, ExtendInvitationsForm, was added to define a form specific for extending invitations. This should streamline the process for administrators and make the interface more user-friendly.

• New Logic for Invitation Handling
  The changes introduced also improve how the system handles and processes invitation extension, including updating the invitation expiration dates. Additionally, better error handling and messaging have been added to keep administrators informed.

• New Template for User Interface
  Finally, a new HTML file was added. This is a template file tasked with presenting the above changes in a visually pleasing and clear way, improving the overall user interface. This file paints the application's functionality for extending invitations onto the screen for an admin to interact with.