support generic key/value fields

[maddingue]

Currently, the user information is mostly in the "user", with a few bits in the "participations" table (like the tshirt size). Adding new fields means updating the database schema, and applies to all conferences.

There is therefore a need for a generic key/value system, which should ideally be configurable by the conference organizers, from act.ini as a good start, through a web interface.