Use Canonical References from Task to ActivityDefinition

[alexanderkiel]

Currently, Task resources use the instantiatesUri property to refer to the ActivityDefinition by an URI that appends the version of the ActivityDefinition as path at the end of its real URI. The custom regex INSTANTIATES_URI_PATTERN in class TaskAuthorizationRule is used to separate the version number from the real URI.

However the canonical data type is supposed to support exactly that, the separation from URI and version. So I would suggest to use instantiatesCanonical instead.

I came up with this issue after looking for a reason why ActivityDefinition URI's have to start with http://highmed.org/bpe/Process, because I would like to use something else for the CODEX ones.

I'm happy to create a PR if this issue is considered valid.