Refactor placeholder logo into common controller (#673)

This can be updated to redirect/pull from third party sources in future,
with the option of always falling back to the placeholder if there are
any failures.

app/views/accounts/logos/show.svg.erb

@@ -0,0 +1,21 @@
+<svg
+  version="1.1"
+  xmlns="http://www.w3.org/2000/svg"
+  xmlns:xlink="http://www.w3.org/1999/xlink"
+  viewBox="0 0 32 32"
+  aria-hidden="true">
+  <g>
+    <rect width="100%" height="100%" rx="50%" fill="<%= accountable_color(@account.accountable_type) %>" opacity="0.1" />
+    <text
+      x="50%"
+      y="50%"
+      fill="<%= accountable_color(@account.accountable_type) %>"
+      text-anchor="middle"
+      dy="0.35em"
+      font-family="ui-sans-serif,system-ui,sans-serif,Apple Color Emoji,Segoe UI Emoji,Segoe UI Symbol,Noto Color Emoji"
+      font-size="16"
+      font-weight="400">
+      <%= @account.name[0].upcase %>
+    </text>
+  </g>
+</svg>

app/views/accounts/show.html.erb

@@ -2,9 +2,7 @@
 <div class="space-y-4">
   <div class="flex justify-between items-center">
     <div class="flex items-center gap-3">
-      <div class="bg-green-600/10 rounded-full h-8 w-8 flex items-center justify-center">
-        <span class="text-green-600"><%= @account.name[0].upcase %></span>
-      </div>
+      <%= image_tag account_logo_url(@account), class: "w-8 h-8" %>
       <h2 class="font-medium text-xl"><%= @account.name %></h2>
     </div>
     <div class="flex items-center gap-3">