Add co-speaker feature broken

[lcduong]

This PR closes/references issue #129 . It does so by:

• In Proposal edit, if 'Additional Speaker' is filled, invitation will be send.

How has this been tested?

Checklist

• [x] I have added tests to cover my changes.

Summary by Sourcery

This pull request introduces a new feature that sends an invitation email to an additional speaker when specified in the proposal edit form. It includes error handling for email sending failures.

• New Features:
  • Added functionality to send an invitation email when an 'Additional Speaker' is specified in the proposal edit form.

[sourcery-ai[bot]]

Reviewer's Guide by Sourcery

This pull request implements a feature to send an invitation email to an additional speaker when the 'Additional Speaker' field is filled in the proposal edit form. The changes include adding a check for the 'additional_speaker' field, sending the invitation email, and handling potential email sending errors with appropriate logging and user notifications.

File-Level Changes

Files	Changes
src/pretalx/cfp/views/user.py	Introduced functionality to send an invitation email to an additional speaker when the 'Additional Speaker' field is filled during proposal editing, including error handling and user notifications.

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[mariobehling]

Thanks this solves the core issue. Please also ensure the log message on the proposal page is showing the action correctly.

When the speaker field invites a speaker the log message for the session should show "A speaker was invited to the proposal."

Currently the log message is as follows: