Handle profil pictures for researchers profiles

[nicoSix]

Right now, the frontend has this feature, using a dedicated component in material-ui (profil pictures can be seen in the left navigation pane, the settings and the questions menu). But there is no such feature in backend, and this needs to be implemented:

• Upload a picture in Settings
• Verify the picture in the backend (to avoid any vulnerabilities) and store it
• Download the picture when logging in