This link standardizes how all the other design pages will refer to
future features (either in `FutureFeatures.md` or in a tracking issue).
All links to these upcoming features now use references at the bottom of
the doc page (as opposed to inline links or references).

Note that this change makes it very easy for the doc links to be updated
when a tracking issue is started. As all the references have a standard
format; a simple find/replace script will get everything.

This commit also makes sure that references now point to a tracking
issue if one exists. Note that this means a future CL can now delete
some of the unnecessary sections from `FutureFeatures.md`.

The only visible change from this commit is that certain broken links
are now fixed, or links pointing to a doc section now point to the
correct tracking issue.