improve management of opt-in

[vecna]

this list of improvement is due to our roadmap and suggested by adopter feedback:

• style at the moment the code is injected from app.js instead of having a proper materialUI component rendered in react.
• content maybe should be clear that the opt-in is necessary to let us process the data
• content we should ask the user if she/he is > 18yo
• review from the web-app panel you should opt-out in any moment and put the web-extension dominant
• logic if the user closes the first box, facebook should work, the extension should not, and offer the opt-in the day after. (if the user don't want fbTREX, just uninstall the extension instead of keeping it dormant)

[vecna]

outdated with v2.0.0