re-introduce custom signup forms [3]

[aguestuser]

[stub]

Context

• realized while blocked playing 2-step email signup path [3] #613 that cusom forms are heavy and we don't actually need them yet, so we ditched them until we do
• by "heavy" i mean: it is difficult to introduce non-text fields into the schema (like passwords and dropdowns) and difficult to run migrations from the schema that worked for just text fields
• when groups start asking for different fields in their signup forms or multiple signup forms that allow them to track the source for new signups, then play this

Implementation Notes

• currently we store a FormInputGroup that has a string field for inputs and required which is stored in postgres as an array (ie: :inputs, :string, array: true, default: [] settings in the original migration)
• we could changes this to :inputs, :text, default: '[]', but is requires non-trivial backfills and breaks rollbacks
• instead, i think we should ditch the inputs and required fields altogether and instead introduce a has_many-belongs-to relationship between FormInputGroup and a new class called FormInputGroup
• where FormInputGroup has the fields type (an enum with possible values: text, textarea, select, password, etc.), attr_name (for the attribute the form field will create), label (for the form label), and, optionally options (if it is a select).
• this will allow cleaner logic for aliasing and recongizing required fields and will generally be cleaner than trying to do it with hashes
• it will require slightly rewriting SignupForm.for