homepage coloured as visited in the Graph (#1128)

simplifies slug from FullSlug to SimpleSlug before storing it in the visited pages list in memory
this leads to "index" page and "folder/index", "tags/tag/index" being stored a "/", "folder/" and "tags/tag/" respectively in the list of visited pages.
this ensures that the homepage is rightfully coloured as a visited page in the "color" function of the graph