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

misc/config_definitions.json

@@ -1427,6 +1427,13 @@
                 "value": "smtp"
             }
         },
+        "email_attach_graphs": {
+            "default": true,
+            "group": "alerting",
+            "section": "email",
+            "order": 4,
+            "type": "boolean"
+        },
         "email_backend": {
             "default": "mail",
             "group": "alerting",