Add confirmation step for email changes (#6071)

* Add confirmation step for email changes

This adds a confirmation step for email changes of existing users.
Like the initial account confirmation, a confirmation link is sent
to the new address.

Additionally, a notification is sent to the existing address when
the change is initiated. This message includes instruction to reset
the password immediately or to contact the instance admin if the
change was not initiated by the account owner.

Fixes #3871

* Add review fixes

app/views/user_mailer/email_changed.en.html.erb

@@ -0,0 +1,15 @@
+<p>Hello <%= @resource.email %>!</p>
+
+<% if @resource&.unconfirmed_email? %>
+  <p>We're contacting you to notify you that the email you use on <%= @instance %> is being changed to <%= @resource.unconfirmed_email %>.</p>
+<% else %>
+  <p>We're contacting you to notify you that the email you use on <%= @instance %> has been changed to <%= @resource.email %>.</p>
+<% end %>
+
+<p>
+  If you did not change your email, it is likely that someone has gained access to your account. Please change your password immediately or contact the instance admin if you're locked out of your account.
+</p>
+
+<p>Sincerely,<p>
+
+<p>The <%= @instance %> team</p>

app/views/user_mailer/email_changed.en.text.erb

@@ -0,0 +1,13 @@
+Hello <%= @resource.email %>!
+
+<% if @resource&.unconfirmed_email? %>
+We're contacting you to notify you that the email you use on <%= @instance %> is being changed to <%= @resource.unconfirmed_email %>.
+<% else %>
+We're contacting you to notify you that the email you use on <%= @instance %> has been changed to <%= @resource.email %>.
+<% end %>
+
+If you did not change your email, it is likely that someone has gained access to your account. Please change your password immediately or contact the instance admin if you're locked out of your account.
+
+Sincerely,
+
+The <%= @instance %> team

app/views/user_mailer/reconfirmation_instructions.en.html.erb

@@ -0,0 +1,15 @@
+<p>Hello <%= @resource.unconfirmed_email %>!</p>
+
+<p>You requested a change to the email address you use on <%= @instance %>.</p>
+
+<p>To confirm your new email, please click on the following link:<br>
+<%= link_to 'Confirm my email address', confirmation_url(@resource, confirmation_token: @token) %></p>
+
+<p>If the above link did not work, copy and paste this URL into your address bar: <br>
+<span><%= confirmation_url(@resource, confirmation_token: @token) %></span>
+
+<p>Please also check out our <%= link_to 'terms and conditions', terms_url %>.</p>
+
+<p>Sincerely,<p>
+
+<p>The <%= @instance %> team</p>

app/views/user_mailer/reconfirmation_instructions.en.text.erb

@@ -0,0 +1,12 @@
+Hello <%= @resource.unconfirmed_email %>!
+
+You requested a change to the email address you use on <%= @instance %>.
+
+To confirm your new email, please click on the following link:
+<%= confirmation_url(@resource, confirmation_token: @token) %>
+
+Please also check out our terms and conditions <%= terms_url %>
+
+Sincerely,
+
+The <%= @instance %> team