projects: add capability to list non-OSGeo member contributors

[rouault]

Currently you can only add as core contributors of a project people who have a account in the website. Could be useful to link to github account as well for example

[jodygarnett]

On the other hand I would love to make sure that core contributors all have edit access to fix and correct their project web pages.

You should be able to use the project "contributors" link to reference your github contributors page.