Add "Flag App" element

[friedger]

Examples why users should be able to flag apps could be cases where

• they find apps requesting too many permissions
• they find the app not functioning properly
• they find illegal or offending content in the app
• they find their user data at security risk

This can also be used to flag apps for violations of the app mining rules (e.g https://github.com/blockstack/app-mining/issues/137 or https://github.com/blockstack/app-mining/issues/175)

The backend could be a simple blocksurvey