Clean the answer from the email content

[marien-probesys]

Overview

Today, when a user answers to an email, the initial message is included. This clutters the ticket thread.

I don't want a system too smart that would detect any kind of answers. Let's add some delimiters in the emails around the answer, like:

=-=-=-= To reply, write above =-=-=-=
[initial message]
=-=-=-= To reply, write below =-=-=-=

Then, when we receive and parse the email content, we can remove the text between the delimiters.

We could also include the ticket id in this delimiter, which would solve the last thing that I didn't do in https://github.com/Probesys/bileto/issues/36