Refactor rightmost navbar dropdown to use .is-right

Replaces the custom negative `left` pixel offset (`.is-enterprise`) with
the built-in `.is-right` CSS utility class for the rightmost navigation
dropdown.

Previously, a hardcoded offset was used to roughly center the rightmost
dropdown to prevent its large box-shadow from being clipped by the right
edge of the browser window. However, with the addition of the theme toggle
switch, there is now sufficient natural buffer space on the right side of
the navigation menu. This makes the custom offset unnecessary and allows
us to safely use the cleaner `.is-right` alignment without visual
clipping.

Closes gh-289

Author: rwinch

src/css/header.css

@@ -348,10 +348,6 @@ body {
   padding-top: 40px;
 }
 
-.is-enterprise .navbar-dropdown {
-  left: -140px;
-}
-
 .external-link-icon {
   width: 12px;
   margin-left: 10px;

src/partials/header-content.hbs

@@ -108,7 +108,7 @@
 
         <div class="navbar-item has-dropdown is-hoverable is-enterprise">
           <a class="navbar-link" href="#">Enterprise</a>
-          <div class="navbar-dropdown lg">
+          <div class="navbar-dropdown lg is-right">
             <a class="navbar-item" href="https://enterprise.spring.io/">Overview</a>
             <a class="navbar-item" href="https://enterprise.spring.io/lts-releases">Long-term Support</a>
             <a class="navbar-item" href="https://enterprise.spring.io/spring-application-advisor">Automated Upgrades</a>