Add /blog base to post URLs

[bonbud-macryg]

No description provided.

[ashelkovnykov]

Too much repetition - if we want to change the prefix to something else, or add more to the prefix, we'll need to change six different locations. Should add a helper function that takes a path and produces a path, e.g.:

++  format
  |=  =path
  ^-  ^path
  (spat [~.blog path])

format isn't the best name, but I can't think of anything better right now - you get the idea.