[form] Add a control list of sources for reporting

[karlcow]

When an issue is reported it adds the

<!-- @reported_with: something_here -->

To strengthen a bit the form and make it easier for partners to contribute in more interesting ways, we would be better to have a controlled list of keywords.

Currently we have:

• desktop-reporter (from the report site issue button on Desktop)
• mobile-reporter (from the report site issue button on Android)
• addon-reporter (from webcompat addon instal)
• web (directly from the website)
• something (from another project)

Some of these are inserted through the GET request and the &src=something argument. We need to make it stricter and just accept what we consider legit for the project.

[karlcow]

We probably need to review all of these parameters and see if we can reduce the number of things which are allowed. I'm in the process of documenting the API.