Backend needs to send contract filename

[luciatugui]

Description

In the web app, when trying to download one of the contracts, the file being downloaded has a name similar to this one: d4eff344-eada-4c7b-bd10-21f952d7fc5c-Test which is the name coming from the backend.
Ideally, we would want a custom name, maybe involving the volunteer name.

In the frontend, we are trying to send a custom name for the file, by setting the download attribute of the <a> element:
<a href="something" download="Custom_Contract_Name". The problem is that the download attribute sets the name of the file only in the cases of same-origin, which is not our case, therefore being ignored.

Possible Solution

The backend could sent a Content-Disposition header that specifies a filename.

Issue was first mentioned here: #318 (comment)