Email: Allow configuration of content types for email notifications (#34530)

* Alerting: Allow configuration of content types for email notifications

* Fix lint error

* Improves email templates

* Improve configuration documentation

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

* Improve code comments

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

* Improve configuration documentation

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

* Improve email template

* Remove unnecessary predeclaration

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Adds handling for unrecognized content type

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Move utility function outside of util package

* Fixes syntax

* Remove unused package

* Fix lint error

* improve email templates

* Fix test

* Alerting: Allow configuration of content types for email notifications

* Fix lint error

* Improves email templates

* Improve configuration documentation

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

* Improve code comments

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

* Improve configuration documentation

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

* Improve email template

* Remove unnecessary predeclaration

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Adds handling for unrecognized content type

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Move utility function outside of util package

* Fixes syntax

* Remove unused package

* Fix lint error

* improve email templates

* Fix test

* Fix comment style

Co-authored-by: Ganesh Vernekar <15064823+codesome@users.noreply.github.com>

* Fix template formatting

* Add test and improve error handling

* Fix test

* Fix formatting

* Fix formatting

* Improve documentation and regenerates txt template

* Update docs/sources/administration/configuration.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

Co-authored-by: Djairho Geuens <djairho.geuens@ae.be>
Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>
Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>
Co-authored-by: Ganesh Vernekar <15064823+codesome@users.noreply.github.com>
Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

docs/sources/administration/configuration.md

@@ -909,7 +909,11 @@ Default is `false`.
 
 ### templates_pattern
 
-Default is `emails/*.html`.
+Enter a comma separated list of template patterns. Default is `emails/*.html, emails/*.txt`.
+
+### content_types
+
+Enter a comma-separated list of content types that should be included in the emails that are sent. List the content types according descending preference, e.g. `text/html, text/plain` for HTML as the most preferred. The order of the parts is significant as the mail clients will use the content type that is supported and most preferred by the sender. Supported content types are `text/html` and `text/plain`. Default is `text/html`.
 
 <hr>
 

docs/sources/http_api/admin.md

@@ -97,8 +97,9 @@ Content-Type: application/json
     "user":"root"
   },
   "emails":{
-    "templates_pattern":"emails/*.html",
-    "welcome_email_on_sign_up":"false"
+    "templates_pattern":"emails/*.html, emails/*.txt",
+    "welcome_email_on_sign_up":"false",
+    "content_types":"text/html"
   },
   "log":{
     "buffer_len":"10000",