Rewrite Python comments in Google developer documentation style (#12)

* Rewrite comments in Google developer documentation style

Rewrite the comments and docstrings across the backend core modules so they
read plainly. The previous prose was accurate but dense and figurative, which
made it slow to skim.

Applies the Google developer documentation style guide: short sentences, active
voice, present tense, American spelling, and no metaphors, idioms, or
rhetorical asides. Replaces em-dash chains with separate sentences.

backend/seed_demo.py

@@ -19,7 +19,8 @@ migrations.bootstrap(engine)
 DEMO_PREFIX = "[Demo]"
 
 # ---------------------------------------------------------------------------
-# Sample scripts — AI Dungeon contract: define modifier(text), call it last.
+# Sample scripts. The AI Dungeon contract is to define `modifier(text)` and call
+# it last.
 # ---------------------------------------------------------------------------
 
 DICE_ROLLER = dict(
@@ -227,7 +228,7 @@ try:
     scripts = [models.Script(**s) for s in SCRIPTS]
     db.add_all(scripts)
 
-    # Scenario with cards and scripts attached — public starter content.
+    # A scenario with cards and scripts attached, as public starter content.
     scenario = models.Scenario(**SCENARIO, is_public=True)
     scenario.scripts = scripts
     db.add(scenario)