user upgrades: add upgrade code system.

Add a system for upgrading accounts using upgrade codes. Users purchase
an upgrade code off-site then redeem it on-site to upgrade their account
to Gold. Upgrade codes are randomly pre-generated and are one time use
only. Codes have enough randomness that guessing a code is infeasible.

app/views/upgrade_codes/index.html.erb

@@ -0,0 +1,29 @@
+<div id="c-upgrade-codes">
+  <div id="a-index">
+    <%= search_form_for(upgrade_codes_path) do |f| %>
+      <%= f.input :redeemer_name, label: "Redeemer", input_html: { value: params[:search][:redeemer_name], data: { autocomplete: "user" } } %>
+      <%= f.input :status, collection: UpgradeCode.statuses, include_blank: true, selected: params[:search][:status] %>
+      <%= f.submit "Search" %>
+    <% end %>
+
+    <%= table_for @upgrade_codes, class: "striped autofit" do |t| %>
+      <% t.column :code do |user_upgrade| %>
+        <%= tag.span user_upgrade.code, class: "font-monospace" %>
+      <% end %>
+
+      <% t.column "Redeemer" do |upgrade_code| %>
+        <% if upgrade_code.redeemer.present? %>
+          <%= link_to_user(upgrade_code.redeemer) %>
+        <% end %>
+      <% end %>
+
+      <% t.column :status %>
+
+      <% t.column "Updated" do |upgrade_code| %>
+        <%= time_ago_in_words_tagged(upgrade_code.updated_at) %>
+      <% end %>
+    <% end %>
+
+    <%= numbered_paginator(@upgrade_codes) %>
+  </div>
+</div>

app/views/upgrade_codes/redeem.html.erb

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+<% page_title "Redeem upgrade code" %>
+
+<%= render "users/secondary_links" %>
+
+<div id="c-user-upgrades">
+  <div id="a-redeem" class="fixed-width-container mx-auto">
+    <h1 class="text-center mb-4">Redeem upgrade code</h1>
+
+    <% if CurrentUser.user.level <= User::Levels::MEMBER %>
+      <p>Enter your upgrade code below to upgrade your account to Gold.</p>
+
+      <p>After you purchase a Gold account, you will receive an upgrade code. Enter it here to upgrade your account. If you don't have a code,
+      go to the <%= link_to "upgrade page", new_user_upgrade_path %> first to buy one, then come back here to redeem it.</p>
+
+      <% if CurrentUser.user.is_anonymous? %>
+        <p>You must <%= link_to "login", login_path(url: redeem_upgrade_codes_path) %> or
+        <%= link_to "create a new account", new_user_path(url: redeem_upgrade_codes_path) %> first before you can redeem your upgrade code.</p>
+      <% else %>
+        <%= simple_form_for(:upgrade_code, url: upgrade_upgrade_codes_path, remote: true) do |f| %>
+          <%= f.input :code, placeholder: "abcd1234", input_html: { value: params[:code] } %>
+          <%= f.submit "Upgrade account", class: "button-primary" %>
+        <% end %>
+      <% end %>
+    <% else %>
+      <p class="text-center">Your account has already been upgraded.</p>
+    <% end %>
+  </div>
+</div>

app/views/upgrade_codes/upgrade.js.erb

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+window.location.assign("<%= j user_upgrade_path(@upgrade_code.user_upgrade) %>");