adding a cmf_is_linkable twig function / templating helper

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  |yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | TODO

I first built that function in a project but it comes in quite handy in many places. Helps to survive incomplete data that can happen when content is being edited.