update icon/favicon (#9)

Related to #417, #433 and #436.

I used https://realfavicongenerator.net to generate multiple favicon files of the new design by @timonss in different formats. These are the previews:

![android](https://user-images.githubusercontent.com/6628437/41185230-25aa4292-6b7e-11e8-9bb4-714c443eeefb.png) ![ios](https://user-images.githubusercontent.com/6628437/41185232-2733a9be-6b7e-11e8-97f6-ae8915f21980.png)
![safari](https://user-images.githubusercontent.com/6628437/41185235-28bed84e-6b7e-11e8-9b45-95a227bebbe4.png) ![win](https://user-images.githubusercontent.com/6628437/41185237-2a379904-6b7e-11e8-918d-ea600d8704a4.png)

static/img/icons/browserconfig.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<browserconfig>
+    <msapplication>
+        <tile>
+            <square150x150logo src="/mstile-150x150.png"/>
+            <TileColor>#455a64</TileColor>
+        </tile>
+    </msapplication>
+</browserconfig>

static/manifest.json

@@ -16,5 +16,5 @@
   "start_url": "{{ .BaseURL }}/",
   "display": "standalone",
   "background_color": "#ffffff",
-  "theme_color": "#2979ff"
+  "theme_color": "#455a64"
 }

static/share/404.html

@@ -9,13 +9,13 @@
   <link rel="icon" type="image/png" sizes="16x16" href="{{ .BaseURL }}/static/img/icons/favicon-16x16.png">
   <!--[if IE]><link rel="shortcut icon" href="{{ .BaseURL }}/static/img/icons/favicon.ico"><![endif]-->
   <link rel="manifest" href="{{ .BaseURL }}/static/manifest.json">
-  <meta name="theme-color" content="#2979ff">
+  <meta name="theme-color" content="#455a64">
   <meta name="apple-mobile-web-app-capable" content="yes">
   <meta name="apple-mobile-web-app-status-bar-style" content="black">
   <meta name="apple-mobile-web-app-title" content="assets">
-  <link rel="apple-touch-icon" href="{{ .BaseURL }}/static/img/icons/apple-touch-icon-152x152.png">
-  <meta name="msapplication-TileImage" content="{{ .BaseURL }}/static/img/icons/msapplication-icon-144x144.png">
-  <meta name="msapplication-TileColor" content="#2979ff">
+  <link rel="apple-touch-icon" href="{{ .BaseURL }}/static/img/icons/apple-touch-icon.png">
+  <meta name="msapplication-TileImage" content="{{ .BaseURL }}/static/img/icons/mstile-144x144.png">
+  <meta name="msapplication-TileColor" content="#455a64">
 
   <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/normalize/7.0.0/normalize.min.css">
   <style>

static/share/index.html

@@ -9,13 +9,13 @@
   <link rel="icon" type="image/png" sizes="16x16" href="{{ .BaseURL }}/static/img/icons/favicon-16x16.png">
   <!--[if IE]><link rel="shortcut icon" href="{{ .BaseURL }}/static/img/icons/favicon.ico"><![endif]-->
   <link rel="manifest" href="{{ .BaseURL }}/static/manifest.json">
-  <meta name="theme-color" content="#2979ff">
+  <meta name="theme-color" content="#455a64">
   <meta name="apple-mobile-web-app-capable" content="yes">
   <meta name="apple-mobile-web-app-status-bar-style" content="black">
   <meta name="apple-mobile-web-app-title" content="assets">
-  <link rel="apple-touch-icon" href="{{ .BaseURL }}/static/img/icons/apple-touch-icon-152x152.png">
-  <meta name="msapplication-TileImage" content="{{ .BaseURL }}/static/img/icons/msapplication-icon-144x144.png">
-  <meta name="msapplication-TileColor" content="#2979ff">
+  <link rel="apple-touch-icon" href="{{ .BaseURL }}/static/img/icons/apple-touch-icon.png">
+  <meta name="msapplication-TileImage" content="{{ .BaseURL }}/static/img/icons/mstile-144x144.png">
+  <meta name="msapplication-TileColor" content="#455a64">
 
   <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/normalize/7.0.0/normalize.min.css">
   <style>