Replace the create_context() and free_context() class methods with a
"context_size" class member defaulting to sizeof(EMailFormatterContext).

EMailFormatter will use "context_size" to allocate a zero-filled slab of
heap memory.  Since EMailFormatterQuote is currently the only thing that
overrides the "context_size" (to append a "qf_flags" member), let's keep
this simple.