HTMLForm: Allow i18n of 'options'

One shortcoming in HTMLForm is that fields that use the 'options'
parameter (e.g. <select>) have no easy way for the individual labels to
be localized.

This change adds a new parameter type, 'options-messages', where the
keys are message keys rather than bare strings (similar to the
difference between 'label' and 'label-message'). It also abstracts out
the fetching of the various option parameters, and changes the necessary
field classes to use it.

Change-Id: If4175332405d26c7ff2e8fbe100bcad61762ce6f