Reviewer: make play_html method aware of side

This is not currently used, but will be usable for applying different
rules to the different sides.
Split the from_template Sanitizer

This creates from_template_front and from_template_back so that they can
be separately controlled (e.g. for cloze deletion handling).
Introduce "only read revealed text" cloze option

This introduces a new checkbox to the configuration screen for
configuring a special on-the-fly mode where only the revealed cloze text
is read.
Support for reading only revealed cloze in OTF

Adds a new 'clozes_revealed' rule for handling the case where the user
only wants the revealed text to be read.