Add responsiveness for footer. Update gulpjs, package.json and bootstrap sass

static/assets/bootstrap/sass/_nav.sass

@@ -0,0 +1,99 @@
+// Base class
+//
+// Kickstart any navigation component with a set of style resets. Works with
+// `<nav>`s or `<ul>`s.
+
+.nav
+  display: flex
+  flex-wrap: wrap
+  padding-left: 0
+  margin-bottom: 0
+  list-style: none
+
+.nav-link
+  display: block
+  padding: $nav-link-padding-y $nav-link-padding-x
+
+  +hover-focus
+    text-decoration: none
+
+
+  // Disabled state lightens text
+  &.disabled
+    color: $nav-link-disabled-color
+
+//
+// Tabs
+//
+
+.nav-tabs
+  border-bottom: $nav-tabs-border-width solid $nav-tabs-border-color
+
+  .nav-item
+    margin-bottom: -$nav-tabs-border-width
+
+  .nav-link
+    border: $nav-tabs-border-width solid transparent
+
+    +border-top-radius($nav-tabs-border-radius)
+
+    +hover-focus
+      border-color: $nav-tabs-link-hover-border-color
+
+
+    &.disabled
+      color: $nav-link-disabled-color
+      background-color: transparent
+      border-color: transparent
+
+  .nav-link.active,
+  .nav-item.show .nav-link
+    color: $nav-tabs-link-active-color
+    background-color: $nav-tabs-link-active-bg
+    border-color: $nav-tabs-link-active-border-color
+
+  .dropdown-menu
+    // Make dropdown border overlap tab border
+    margin-top: -$nav-tabs-border-width
+
+    // Remove the top rounded corners here since there is a hard edge above the menu
+    +border-top-radius(0)
+
+//
+// Pills
+//
+
+.nav-pills
+  .nav-link
+    +border-radius($nav-pills-border-radius)
+
+  .nav-link.active,
+  .show > .nav-link
+    color: $nav-pills-link-active-color
+    background-color: $nav-pills-link-active-bg
+
+//
+// Justified variants
+//
+
+.nav-fill
+  .nav-item
+    flex: 1 1 auto
+    text-align: center
+
+.nav-justified
+  .nav-item
+    flex-basis: 0
+    flex-grow: 1
+    text-align: center
+
+// Tabbable tabs
+//
+// Hide tabbable panes to start, show them when `.active`
+
+.tab-content
+  > .tab-pane
+    display: none
+
+  > .active
+    display: block