DEV: Replace instances of Discourse.base_uri with Discourse.base_path
This is clearer because the base_uri is actually just a path prefix. This continues the work started in 555f467.
This change refactors the code a bit so that a plugin could easily
replace which badge is awarded when completing the discobot new user
tutorial and advanced tutorial.
By adding a static method and putting the BADGE_NAME constant inside of
that method we can simply call that method now instead of the constant.
A plugin could then `class_eval` that method and replace it with
whatever badge name they choose. This is way cleaner than having the
plugin change the frozen constant! eeek.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
