ENH: Only include supported language and file types based on report type selection

[fletchapin]

Use Case

When selecting what kind of report for the website to produce, some file types and languages are only supported by certain report types. For example, the Design Specs can be output in PDF or HTML form, but the validation report can only be a PDF. See this PR and associated issue.

Solution

Ideally, the user selects Report Type first before the Language and File Type options are displayed. This way, only file types supported by the selected report will be displayed in the dropdown.

Alternatives

To avoid this, we put all permutations into a single dropdown, e.g. Design Specs - PDF (Spanish). This single list approach is less user friendly and will grow quite rapidly as new languages or file types are added, so ideally it should be replaced.

[fletchapin]

Oliver mentioned: "The ideal fix you mentioned could probably be implemented in class DocGenForm"