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>

conf/sample.ini

@@ -578,7 +578,8 @@
 
 [emails]
 ;welcome_email_on_sign_up = false
-;templates_pattern = emails/*.html
+;templates_pattern = emails/*.html, emails/*.txt
+;content_types = text/html
 
 #################################### Logging ##########################
 [log]