Add explicit cross-project references to project and group wikis in GLFM

Proposal

Currently we support GitLab-specific references to wiki pages in the [[wiki-page]] format (historically inspired by compatibility with the gollum's tag syntax), which point to group wikis only when used in epics, and to project wikis in all other work item contexts.

We should be able to explicitly reference particular group or project wikis, and be able to do it cross-project, e.g. reference group2's or project2's wiki in group1 or project1 in different namespaces.

Example syntax

[[namespace/project:foo/bar]] -> page foo/bar in project wiki for namespace/project, regardless of the current namespace.