Implement flexbox layout for portfolio grid

Make responsive layout even more responsive.
Support portfolio grids with arbitrary numbers of items.
Previously, I would only add portfolio items in groups of three.

_includes/css/main.css

@@ -252,6 +252,33 @@ section.success {
     }
 }
 
+/* Flexbox Portfolio Grid */
+@media (min-width: 640px) {
+    .portfolio-flex-grid .portfolio-flex-row {
+      display: flex;
+      flex-flow: row wrap;
+      align-items: stretch;
+      justify-content: space-between;
+    }
+    .portfolio-flex-grid .portfolio-flex-row .portfolio-flex-item {
+      flex: 1 0 49%;
+      max-width: 49%;
+    }
+  }
+  @media (min-width: 960px) {
+    .portfolio-flex-grid .portfolio-flex-row .portfolio-flex-item {
+      flex: 1 0 32%;
+      max-width: 32%;
+    }
+  }
+  /* A large grid might need four columns on widescreen layouts */
+  /* @media (min-width: 1380px) {
+    .portfolio-flex-grid .portfolio-flex-row .portfolio-flex-item {
+      flex: 1 0 23.5%;
+      max-width: 23.5%;
+    }
+  } */
+
 .btn-outline {
     margin-top: 15px;
     border: solid 2px #fff;

_includes/portfolio_grid.html

@@ -1,16 +1,16 @@
 
     <!-- Portfolio Grid Section -->
     <section id="portfolio">
-        <div class="container">
+        <div class="container portfolio-flex-grid">
             <div class="row">
                 <div class="col-lg-12 text-center">
                     <h2>Portfolio</h2>
                     <hr class="star-primary">
                 </div>
             </div>
-            <div class="row">
+            <div class="portfolio-flex-row">
                 {% for post in site.posts %}
-                    <div class="col-sm-4 portfolio-item">
+                    <div class="portfolio-flex-item portfolio-item">
                         <a href="#portfolioModal-{{ post.modal-id }}" class="portfolio-link" data-toggle="modal">
                             <div class="caption">
                                 <div class="caption-content">