Add a datatype context tag for templates, and use it during LFConversion.

[sofiafaro-da]

For interfaces and exceptions we add DatatypeContexts/"stupid theta" tags during parsing/desugaring so we can identify them later during LFConversion:

• exception E desugars to data DamlException => E
• interface I desugars to data DamlInterface => I

But for historical reasons we don't do that with templates. Instead, template types are unmarked, and we detect them based on the presence of a HasSignatory instance. We're now running into issues with this approach, because we want to implement HasSignatory for interfaces. It would be nice if, instead, we desugared templates the same way:

• template T desugars to data DamlTemplate => T

And then only use that in LFConversion to detect template types. This would be a damlc-internal change. It does not affect Daml LF, in any way, and is backward-compatible with Daml.