Add prefix g or p in the wiki indices for id

The issue is two documents could have the same id. Consider a case where there is a project wiki with path home.md under the project with id: 5 and there is a group wiki with path home.md under the group with id: 5. Now both wikis will have the id: 5_home.md. The solution is to add a prefix p_ if it is a project wiki and g_ if it is a group wiki.

Closes #140 (closed)