[Users] Backports

Hi,

There was some discussion in https://trac.einsteintoolkit.org/ticket/1504 about how to mark Einstein Toolkit tickets as requiring a backport to the current release. We already introduced a method for this in the past; if you put the keyword "backport" in the keywords field, the tickets requiring backports can be queried using a standard TRAC query, e.g.

https://trac.einsteintoolkit.org/query?status=accepted&status=assigned&a...

In the above query, I have included closed tickets, as there are no current open tickets with the backport keyword.

I have added a link on the ET TRAC homepage which shows the number of open tickets with the backport keyword, and provides a link to the query. I think this should be sufficient.