Resolve: Add option to email take-home exams

Closes #430

This merge request adds the option to send personal copies of an exam by email to all the students in a course.

Frontend:

• New panel in Exam page, allows to select the start copy number and go to Email Edit model.
• Email Model let's the user change the subject and content, this last one is a jinja2 template.

Backend:

• Redistribution of the pdf generation class, included a method that generates a personal copy with student id and name writen in the student id widget.
• Template api now supports two different types of templates, for sending copies and solutions.
• Email api supports sending two types of mails, for copies and solutions.

Example data:

• Use new method in pdf_generation to fill the id widget, consequently the student name is also writen.