Install now submits the proper database name and is properly set using
the config.Value class. This extends the getter functionality so now
config.Value can be used to both get and set values.
Frontend still interacts directly with the database entry name
`picture.disable_gravatar` so logic needs flipped when writing, but
logic to read automatically flips based on config.Invert() being called
during init or INI read.
This PR reduces the complexity of the system setting system.
It only needs one line to introduce a new option, and the option can be
used anywhere out-of-box.
It is still high-performant (and more performant) because the config
values are cached in the config system.
