Using email and/or password fields under form instead of account when only one is present

[stuartpb]

Honestly, I'm pretty sure this is the right call to make: account.accepts should never have one item (or maybe only when it's something esoteric like domain, and even then, I'm not sure).

I mean, at that point, it's not really an account input so much as it is whatever the one value type it accepts is, and it'd be better if forms just reflected that (especially as you might have email-or-username-input-specific properties attached, like "does this have type="email" set). Right now, we have the awkward situation of "dedicated inputs are only used when both are presented", and it leads to something that should be the primary check (checking if the input for the specific data type is required, then checking if it's under account) becoming the edge case. Having this all be one field feels like it's pretty much yet another holdover from the days of accepts fields before form, and one that should be outmoded with a clearer embrace of form (and #169 to solve for the complexity).

[stuartpb]

Yeah, I'm ready to go ahead with this.