Allow null readings to be stored in the database

The latest firmware will return the string "None" if a sensor is faulty.
(This is different to the python value None, which is returned when a
controller is missing.)

Store these readings in the database as null values, so that periods of
the sensor being faulty can be shown on the graph.

Requires a database update:
alter table datalog_floatdatum alter column data drop not null;
alter table datalog_integerdatum alter column data drop not null;
alter table datalog_stringdatum alter column data drop not null;
Add models for notes

The intent here is that notes about what's in the fermenting vessels,
and gravity readings, can be entered and will be shown on graphs.
Primitive graph-drawing code

Generate SVG graphs from logged data.  Only works with float datatype at
the moment.  No axes, labels, etc. yet!
Draw simple graph on fermenter detail pages