Email Transport: embed graphs by default (#14270)

* Email embed graphs

* Allow attachment for non-html
Add setting to webui
Correct $auth setting

* Cleanups, throw RrdGraphException instead of returning an error image.
Generate the error image later, giving more control.
Reduce code duplication a little

* Style and lint fixes
Change to flags

* Add baseline for lint errors I don't know how to resolve

* oopsie, changed the code after generating the baseline

* Tiny cleanups.  Make set DeviceCache primary, it is free.

* Docs.

* email_html note

* Allow control of graph embed at the email transport level to override the global config.

* Allow control of graph embed at the email transport level to override the global config.

* Add INLINE_BASE64 to make it easier to create inline image tags

resources/lang/en/settings.php

@@ -625,6 +625,10 @@ return [
             'description' => 'Auto TLS support',
             'help' => 'Tries to use TLS before falling back to un-encrypted',
         ],
+        'email_attach_graphs' => [
+            'description' => 'Attach graph images',
+            'help' => 'This will generate a graph when the alert is raised and attach it and embed it in the email.',
+        ],
         'email_backend' => [
             'description' => 'How to deliver mail',
             'help' => 'The backend to use for sending email, can be mail, sendmail or SMTP',