fix: back button icon rendering inconsistently (#2306)

Depending on the operating system and therefore the system font
the back icon being a unicode arrow means it'll render inconsistently,
sometimes I've seen it looking really odd.

Instead make use of the font awesome arrow so that'll it render consistently
no matter ths system font.

bin/svgs.js

@@ -1,5 +1,6 @@
 export default [
   { id: 'pinafore-logo', src: 'src/static/sailboat.svg', inline: true },
+  { id: 'fa-arrow-left', src: 'src/thirdparty/font-awesome-svg-png/white/svg/arrow-left.svg' },
   { id: 'fa-bell', src: 'src/thirdparty/font-awesome-svg-png/white/svg/bell.svg', inline: true },
   { id: 'fa-bell-o', src: 'src/thirdparty/font-awesome-svg-png/white/svg/bell-o.svg' },
   { id: 'fa-users', src: 'src/thirdparty/font-awesome-svg-png/white/svg/users.svg', inline: true },

src/routes/_components/DynamicPageBanner.html

@@ -8,7 +8,10 @@
   <button type="button"
           class="dynamic-page-go-back"
           aria-label="{intl.goBack}"
-          on:click|preventDefault="onGoBack()">{intl.back}</button>
+          on:click|preventDefault="onGoBack()">
+    <SvgIcon className="dynamic-page-go-back-icon" href="#fa-arrow-left" />
+    {intl.back}
+  </button>
 </div>
 <Shortcut key="Backspace" on:pressed="onGoBack()"/>
 <style>
@@ -34,19 +37,25 @@
     text-overflow: ellipsis;
   }
   .dynamic-page-go-back {
-    font-size: 1.3em;
+    display: inline-flex;
+    align-items: center;
+    justify-self: flex-end;
+    font-size: 1.2857142857142858em;
     color: var(--anchor-text);
     border: 0;
     padding: 0;
     background: none;
-    justify-self: flex-end;
   }
   .dynamic-page-go-back:hover {
     text-decoration: underline;
   }
-  .dynamic-page-go-back::before {
-    content: '←';
-    margin-right: 5px;
+  :global(.dynamic-page-go-back-icon) {
+    position: relative;
+    bottom: 0.06em;
+    margin-right: 0.2em;
+    height: 0.66666666em;
+    width: 0.66666666em;
+    fill: currentColor;
   }
   @media (max-width: 767px) {
     .dynamic-page-banner {