Make registration of ILink serializer conditional

[erral]

Fixes #1117

[mister-roboto]

@erral thanks for creating this Pull Request and help improve Plone!

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass.

Whenever you feel that the pull request is ready to be tested, either start all jenkins jobs pull requests by yourself, or simply add a comment in this pull request stating:

@jenkins-plone-org please run jobs

With this simple comment all the jobs will be started automatically.

Happy hacking!

[erral]

@jenkins-plone-org please run jobs

[davisagli]

@erral This PR is out of date and has a number of failing builds. Do you want to update it, or close it?

[davisagli]

I think this is out of date (relevant to Plone 4 only). I'll close it.