Fill `ModelMetadata` gaps needed for HTML helpers

[dougbu]

The HTML helpers aren't completely copied over but already contain TODO items for missing ModelMetadata properties:

• DisplayName, which is particularly important
• HideSurroundingHtml, the opposite of [Hidden] attribute’s DisplayValue property
• SimpleDisplayText (and protected virtual GetSimpleDisplayText() method), which @sornaks has already begun adding
• ShowForDisplay, which we might need to skip since [ScaffoldColumnAttribute] doesn't seem to exist (yet?)

In addition the default editor templates need:

• HasNonDefaultEditFormat
• ShowForEdit, with same caveat as ShowForDisplay

And finishing validation will require:

• IsRequired !!

[JaneZhouQ]

MusicStore requires DisplayName and ScaffoldColumn.

[sornaks]

DisplayName is already present - 7273286

[JaneZhouQ]

@sornaks, could you please update #532 and assign that issue to @kirthik to validate?

[sornaks]

@JaneZhouQ - Just discussed with @dougbu . As he has mentioned in the other issue, there is a TODO for this item to be complete.