Refactor New Portfolio page according to designs.

New designs call for a streamlined New Portfolio page, with far
fewer input options. This commit refactors that page according to those
designs.

Some of the route functions in this commit refer to a "step 1" of creating
a new Portfolio. Though there is no "step 2" right now, the designs call
for a multistep flow for Portfolio creation process, so this commit sets
the stage for that.

templates/home.html

@@ -14,7 +14,7 @@
   {% endif %}
 
   {% call StickyCTA(sticky_header) %}
-      <a href="{{ url_for("portfolios.new_portfolio") }}" class="usa-button-primary">
+      <a href="{{ url_for("portfolios.new_portfolio_step_1") }}" class="usa-button-primary">
         {{ "home.add_portfolio_button_text" | translate }}
       </a>
   {% endcall %}