Add `DisplayName` property to `ModelMetadata`

See individual commits for details
- note related fixes for `
- test affected HTML helpers thoroughly -- not just with `[DisplayName]` in the mix
- `[DisplayName]` doesn't affect `@Html.Name()`
 - included these tests because they're similar to those for `@Html.DisplayName()`

/cc @pranavkm @sornaks @yishaigalatzer