In emails, style implicit links too

Fixes issue where, when some email clients (macOS/iOS Mail.app)
automatically turn text into links, the links had different styling than
other (explicit) links.

Changelog: changed

app/assets/stylesheets/notify_enhanced.scss

@@ -32,10 +32,6 @@ body {
   font-size: inherit;
 }
 
-a {
-  text-decoration: none;
-}
-
 .gl-mb-5 {
   @include gl-mb-5;
 }

app/views/layouts/notify.html.haml

@@ -5,6 +5,10 @@
       GitLab
     - if Feature.enabled?(:enhanced_notify_css)
       = stylesheet_link_tag 'notify_enhanced'
+      %style{ type: 'text/css', 'data-premailer': 'ignore' }
+        -# The MUA automatically turns some text into links.
+        -# Match the color of explicit links ($blue-600 from typography.scss).
+        a { color: #1068bf; }
     - else
       = stylesheet_link_tag 'notify'
     = yield :head

app/views/layouts/service_desk.html.haml

@@ -7,6 +7,10 @@
     -# haml-lint:enable NoPlainNodes
     - if Feature.enabled?(:enhanced_notify_css)
       = stylesheet_link_tag 'notify_enhanced'
+      %style{ type: 'text/css', 'data-premailer': 'ignore' }
+        -# The MUA automatically turns some text into links.
+        -# Match the color of explicit links ($blue-600 from typography.scss).
+        a { color: #1068bf; }
     - else
       = stylesheet_link_tag 'notify'
     = yield :head