User message HTML email notifications footer is not rendered

[gileri]

Email notifications used to have a footer if I remember correctly, and that seems in line with the template.

But for at least a few days, the HTML part is truncated and don't include the footer. The plaintext version works fine however :

----==_mimepart_5f91832ca5928_e0176269818620b2
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: quoted-printable

Bonjour gileri,=0D
=0D
bot_gileri vous a envoy=C3=A9 un message depuis OpenStreetMap avec le suj=
et test=C2=A0:=0D
=0D
=3D=3D=0D
test=0D
=3D=3D=0D
=0D
Vous pouvez aussi lire le message sur=0D
https://www.openstreetmap.org/messages/897959 ou envoyer un message =C3=A0=
 l'auteur=0D
sur https://www.openstreetmap.org/messages/897959/reply=0D

----==_mimepart_5f91832ca5928_e0176269818620b2
Content-Type: text/html;
 charset=UTF-8
Content-Transfer-Encoding: quoted-printable

<p>=0D
  Bonjour gileri,=0D
</p>=0D
<p>=0D
  <a target=3D"_blank" rel=3D"noopener" style=3D"text-decoration: none; c=
olor: #222" href=3D"https://www.openstreetmap.org/user/bot_gileri"><stron=
g style=3D"text-decoration: none">bot_gileri</strong></a> vous a envoy=C3=
=A9 un message depuis OpenStreetMap avec le sujet <em>test</em>=C2=A0:=0D=

</p>=0D
=0D
<table style=3D"font-size: 15px; margin: 15px 0px; background-color: #eee=
; width: 100%">=0D
  <tr>=0D
    <td style=3D"width: 50px; min-width: 50px; vertical-align: top; paddi=
ng: 15px">=0D
      <a target=3D"_blank" rel=3D"noopener" href=3D"https://www.openstree=
tmap.org/user/bot_gileri"><img alt=3D"bot_gileri" title=3D"bot_gileri" wi=
dth=3D"50" height=3D"50" border=3D"0" src=3D"cid:5f91832ca3b00_e017626981=
8619e8@spike-07.openstreetmap.org.mail" /></a>=0D
    </td>=0D
    <td style=3D"text-align: left; vertical-align: top; padding-right: 10=
px; width: 100%">=0D
      =0D
  <p>test</p>=0D
=0D
=0D
    </td>=0D
  </tr>=0D
</table>=0D
=0D

----==_mimepart_5f91832ca5928_e0176269818620b2--

[gravitystorm]

This is a bug I introduced in #2884 because I didn't rename the layout template for the notification emails. The template (in app/views/layouts/ ) is still called notifier.html.erb and needs to be renamed to user_mailer.html.erb

We also need to add a test, to make sure this doesn't happen again!

I will fix this in a couple of days, but if anyone else wants to make a PR first, that would be appreciated.