HTML helper refactoring

- Enable optional parameters for FileLinkFor, HyperlinkFor, and ImageFor
- For example, @Html.HyperlinkFor(x=>x.FooField, cssClass: "foo");
- Should be backwards compatible
Return null in case of no items instead an error.

Return a null in
Synthesis.FieldTypes.Adapters.AxesAdapter.SelectItems and
Synthesis.FieldTypes.Adapters.AxesAdapter.SelectSingleItem
Add _ in template path starting with number

Added a _ to the namespace parts generated that start with a number.
This happens in the case that the item name starts with a number.