2.0.0 svg download, image optimization

Release notes :

User

• added "Download as SVG" option
• moved graph transform widgets to sidebar
• limit uploaded image to 2048 pixels max on any dimension (in effort of not overflow the cloud machine RAM)

Developer

• clear image array config option once plot created : to free RAM earlier

Breaking :

• refacto properties names on the UserConfig class
• changed envvar name for debugging

Full Changelog: v1.1.1...v2.0.0