Config option for location of site editing instructions

[rufuspollock]

Currently the link for help instructions for editing is hard-coded to contribute however for many sites this is not appropriate.

Suggest we have a new config option editsitehowto and use this _includes/page-tools.html and default it to /contribute/. Not we should not preprend baseurl as this url could be offsite.

[rufuspollock]

Also in addition i suggest we merge the "contribute" section into one large page:

• This will avoid issues where by moving contribute page you break it (the next and back links are location specific)
• Also easier to use - you just have one big page (and the next icon is not obvious)