Use instance name in email notifications instead of "Mastodon" (#3763)

* Use instance name in "password changed" mail

instead of "Mastodon".

Fixes tootsuite#2620.

* Use instance name in password reset mail

instead of "Mastodon".

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

@@ -1,3 +1,3 @@
 <p>Hello <%= @resource.email %>!</p>
 
-<p>We're contacting you to notify you that your password on Mastodon has been changed.</p>
+<p>We're contacting you to notify you that your password on <%= @instance %> has been changed.</p>

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

@@ -1,3 +1,3 @@
 Hello <%= @resource.email %>!
 
-We're contacting you to notify you that your password on Mastodon has been changed.
+We're contacting you to notify you that your password on <%= @instance %> has been changed.

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

@@ -1,6 +1,6 @@
 <p>Hello <%= @resource.email %>!</p>
 
-<p>Someone has requested a link to change your password on Mastodon. You can do this through the link below.</p>
+<p>Someone has requested a link to change your password on <%= @instance %>. You can do this through the link below.</p>
 
 <p><%= link_to 'Change my password', edit_password_url(@resource, reset_password_token: @token) %></p>
 

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

@@ -1,6 +1,6 @@
 Hello <%= @resource.email %>!
 
-Someone has requested a link to change your password on Mastodon. You can do this through the link below.
+Someone has requested a link to change your password on <%= @instance %>. You can do this through the link below.
 
 <%= edit_password_url(@resource, reset_password_token: @token) %>