Update devise error messages

Devise 4.6.0 deprecated the `devise_error_messages!` helper method in
favour if just rendering a partial including the library. [1]

We can (optionally) copy that partial into our codebase, where we can
edit it, and it will be preferentially used over the one in the gem
code. However, the existing helper had no customisations so I didn't go
down that route yet.

[1] https://github.com/heartcombo/devise/blob/main/CHANGELOG.md#460---2019-02-07

app/views/accounts/registrations/edit.html.erb

@@ -1,7 +1,7 @@
 <h2>Edit <%= params[:type] %> <%#= resource_name.to_s.humanize %></h2>
 
 <%= form_for(resource, :as => resource_name, :url => registration_path(resource_name, :type => params[:type]), :html => { :method => :put }) do |f| %>
-  <%= devise_error_messages! %>
+  <%= render "devise/shared/error_messages", resource: resource %>
   <% if params[:type] == 'username' %>
     <p><%= f.label :username %><br />
     <%= resource.can_change_username ? f.text_field(:username) : resource.username %>

app/views/accounts/registrations/new.html.erb

@@ -6,7 +6,7 @@
   <li>Access more problems.</li>
 </ul>
 <%= form_for(resource, :as => resource_name, :url => registration_path(resource_name)) do |f| %>
-  <%= devise_error_messages! %>
+  <%= render "devise/shared/error_messages", resource: resource %>
 
   <div class="field">
     <h3><%= f.label :username %></h3>

app/views/accounts/settings/edit.html.erb

@@ -2,7 +2,7 @@
 <% toolbox_push :back, :back %>
 
 <%= form_for(resource, :as => resource_name, :url => "/accounts/settings/update", :html => { :multipart => true, :method => :put }) do |f| %>
-  <%= devise_error_messages! %>
+  <%= render "devise/shared/error_messages", resource: resource %>
 
   <div class="field">
     <h3><%= f.label :username %></h3>