Remember/recall last-used fields in mass generator

Uses the user preferences object (conf) to save and recall the last
source and destination field that was used with the mass generator.
Consolidate take_a_break()

This can be done in fewer lines then at present, and is only called in
once place, making this merge straight-forward.
Save/recall last voice used

For each service, when the user records either through the one-off
method or the mass generator method, saves the voice. When returning to
either form, displays the last-used voice.

This is done via a JSON-encoded dict stored in the last_voice slot of
the conf object.
Move service text normalization to main module

This relieves all the service implementations the responsibility of
providing their own encoding/decoding/normalization -- it can all be
handled in one place, since it is done the same across the whole system.