Allow maintainers to change the settings of their project

Right now, only the people with the admin rights on the whole patchwork instance can change project settings like its name, website link, SCM link, ...

It'd be could to add an admin page maintainers can access to configure their own project. That's especially important as I plan to add more per-project configuration, like project members, tag handling, maybe even state handling (so people can define their own patch lifecycle), ...