DEV: Add support for Rails 6

Minor fixes to add Rails 6 support to Discourse, we now will boot
with RAILS_MASTER=1, all specs pass

Only one tiny deprecation left

Largest change was the way ActiveModel:Errors changed interface a
bit but there is a simple backwards compat way of working it

app/views/layouts/_head.html.erb

@@ -5,7 +5,7 @@
 <%- end %>
 <% site_apple_touch_icon_url = SiteSetting.site_apple_touch_icon_url %>
 <%- if site_apple_touch_icon_url.present? %>
-<link rel="apple-touch-icon" type="image/png" href="<%= UrlHelper.absolute(site_apple_touch_icon_url) %>">
+<link rel="apple-touch-icon" type="image/png" href="<%= ::UrlHelper.absolute(site_apple_touch_icon_url) %>">
 <%- end %>
 <meta name="theme-color" content="#<%= ColorScheme.hex_for_name('header_background', scheme_id) %>">
 <% if mobile_view? %>

app/views/metadata/opensearch.xml.erb

@@ -4,7 +4,7 @@
   <Description>Search for posts on <%= SiteSetting.title %></Description>
   <Tags>discourse forum</Tags>
   <% site_favicon_url = SiteSetting.site_favicon_url %>
-  <% absolute_site_favicon_url = UrlHelper.absolute(site_favicon_url) %>
+  <% absolute_site_favicon_url = ::UrlHelper.absolute(site_favicon_url) %>
   <% if site_favicon_url =~ /\.ico$/ -%>
     <Image height="16" width="16" type="image/vnd.microsoft.icon"><%= absolute_site_favicon_url %></Image>
   <%- else -%>

app/views/users/password_reset.html.erb

@@ -4,7 +4,7 @@
       <%= @error %>
     </div>
   <%end%>
-  <% if @user.present? and @user.errors.any? %>
+  <% if @user.present? and @user.errors.present? %>
     <div class='alert alert-error'>
       <% @user.errors.full_messages.each do |msg| %>
         <li><%= msg %></li>