Fix #97 - plain text version of devise e-mails

app/views/auth/mailer/confirmation_instructions.text.erb

+Welcome <%= @resource.email %>!
+
+You can confirm your Mastodon account email through the link below:
+
+<%= confirmation_url(@resource, confirmation_token: @token) %>

app/views/auth/mailer/password_change.text.erb

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

app/views/auth/mailer/reset_password_instructions.text.erb

+Hello <%= @resource.email %>!
+
+Someone has requested a link to change your password on Mastodon. You can do this through the link below.
+
+<%= edit_password_url(@resource, reset_password_token: @token) %>
+
+If you didn't request this, please ignore this email.
+Your password won't change until you access the link above and create a new one.