add support for links refactoring for the github backend

Allow links to be updated when page/attachments are moved/renamed

The difficulty with github is that we don't have control over the backend, so the main option is to do it client side.

But, it does not scale if the renamed page are a lot of backlinks.

Or, the rename is made asynchronously but this can lead to inconsistencies.

Another option is to generate unique ids for pages, and to revolve links based on this. But this leads to ugly links that are hard to edit in source mode.