Add preference for configuring sticky navbar

Fixes #1354

src/sass/index.scss

@@ -115,11 +115,14 @@ ul {
   display: flex;
   flex-wrap: wrap;
   box-sizing: border-box;
-  padding-top: 50px;
   margin: auto;
   min-height: 100vh;
 }
 
+body.fixed-nav .container {
+  padding-top: 50px;
+}
+
 .icon-container {
   display: inline;
 }

src/sass/navbar.scss

@@ -3,7 +3,6 @@
 nav {
   display: flex;
   align-items: center;
-  position: fixed;
   background-color: var(--bg_overlays);
   box-shadow: 0 0 4px $shadow;
   padding: 0;
@@ -16,6 +15,10 @@ nav {
   .icon-button button {
     color: var(--fg_nav);
   }
+
+  body.fixed-nav & {
+    position: fixed;
+  }
 }
 
 .inner-nav {

src/sass/profile/_base.scss

@@ -39,7 +39,11 @@
     text-align: left;
     vertical-align: top;
     max-width: 32%;
-    top: 50px;
+    top: 0;
+
+    body.fixed-nav & {
+        top: 50px;
+    }
 }
 
 .profile-result {

src/sass/tweet/thread.scss

@@ -16,8 +16,10 @@
 
 .main-tweet,
 .replies {
-  padding-top: 50px;
-  margin-top: -50px;
+  body.fixed-nav & {
+    padding-top: 50px;
+    margin-top: -50px;
+  }
 }
 
 .main-tweet .tweet-content {