Duplicate markdown rendering with writefreely

[eddsalkield]

web-core depends on github.com/writeas/saturday to implement functions ApplyMarkdown and ApplyBasicMarkdown in posts/render.go. These functions are specific to the operation of writefreely. However, writefreely reimplements these functions as applyMarkdown and applyBasicMarkdown in postrender.go instead of using the implementations here in web-core.

Indeed, there does not exist any writeas repository that depends on either of those functions.

Therefore I suggest dropping the implementation of these functions from web-core. In so doing, we lose the dependency on github.com/writeas/saturday, simplifying the packaging process by reducing the need for bundling.