Cleanup the way I capture sent emails automatically into OrgMode

- Make last email sent available automatically for org-insert-link
- Invoking djr/org-mu4e-capture-next-message or setting djr/org-mu4e-must-capture-message before composing a message will call org-capture after the email was sent successfully (using the capture template from djr/mu4e-org-mode-capture-template-for-sent-email)