When "invite linked dossiers" is checked, the invitation will generate
one avis per dossier (including the linked ones).
So when we retrieve the email send to the expert, and try to match it
with the avis record, we need to specify wich avis (the one for the main
dossier, or for the linked dossier).
Otherwise the test is flaky, and sometimes doesn't match the right avis
with the sent email.
Make it more reusable, by:
- Renaming `attachment/update` to `attachment/edit`
- Refactoring the CSS styles into their own stylesheet
- Allow to specify the 'accept' option
