Clean formatting for email notifications

[rwjack]

Is your feature request related to a problem? Please describe.

Continuation of https://github.com/containrrr/watchtower/issues/815

I doubt people can see the value in a dump of text, such as this one:

Found new fireflyiii/core:latest image (d7123f673c25)
Found new mariadb:latest image (3e87f8bfed4e)
Could not do a head request for "cthulhoo/ttrss-web-nginx:latest", falling back to regular pull.
Reason: registry responded to head request with "404 Not Found", auth: "not present"
Found new cthulhoo/ttrss-web-nginx:latest image (ce5350fada33)
Found new cthulhoo/ttrss-fpm-pgsql-static:latest image (e1627c2a53c1)
Found new cthulhoo/ttrss-fpm-pgsql-static:latest image (e1627c2a53c1)
Found new cthulhoo/ttrss-fpm-pgsql-static:latest image (e1627c2a53c1)
Found new postgres:12-alpine image (07f0565a99b5)
Found new lscr.io/linuxserver/unifi-controller:latest image (24c56d51863e)
Stopping /unifi-controller (3b8ce540a354) with SIGTERM
Creating /unifi-controller
Stopping /ttrss_db (66a1a0e57333) with SIGTERM
Creating /ttrss_db
Stopping /ttrss (d0ca1a51e721) with SIGTERM
Creating /ttrss
Stopping /ttrss_backup (c4c23b1ee450) with SIGTERM
Creating /ttrss_backup
Stopping /ttrss_updater (dfa9509d4f15) with SIGTERM
Creating /ttrss_updater
Stopping /ttrss_nginx (510861d1292e) with SIGTERM
Creating /ttrss_nginx
Stopping /firefly-db-1 (846a9a03dc3a) with SIGTERM
Creating /firefly-db-1

Describe the solution you'd like

A clean default template.

Describe alternatives you've considered

Tried making my own, though since I'm not being familiar with go templating, I ended up with something like this.

Unfortunately, when the environment variable is set, I simply receive no notification email and no error message why the email didn't send. Meanwhile, using the default template works. I get the email, but it's the default unreadable template...

WATCHTOWER_NOTIFICATION_TEMPLATE='
{{- if .Report -}}
  {{- with .Report -}}
    {{- if ( or .Updated .Failed ) -}}
=======================
Watchtower Update Report
=======================
Scanned Containers: {{ len .Scanned }}
Updated Containers: {{ len .Updated }}
Failed Containers: {{ len .Failed }}

{{- if .Updated }}
Updated Containers:
{{- range .Updated}}
- {{.Name}} ({{.ImageName}}): {{.CurrentImageID.ShortID}} updated to {{.LatestImageID.ShortID}}
{{- end }}
{{- end }}

{{- if .Fresh }}
Fresh Containers:
{{- range .Fresh}}
- {{.Name}} ({{.ImageName}}): {{.State}}
{{- end }}
{{- end }}

{{- if .Skipped }}
Skipped Containers:
{{- range .Skipped}}
- {{.Name}} ({{.ImageName}}): {{.State}}: {{.Error}}
{{- end }}
{{- end }}

{{- if .Failed }}
Failed Containers:
{{- range .Failed}}
- {{.Name}} ({{.ImageName}}): {{.State}}: {{.Error}}
{{- end }}
{{- end }}

{{- end -}}
  {{- end -}}
{{- else -}}
=========================
Watchtower Log Entries
=========================
{{ range .Entries -}}{{.Message}}{{"\n"}}{{- end }}
{{- end -}}
'

Additional context

No response

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