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>

emails/grunt/replace.js

@@ -1,7 +1,7 @@
 module.exports = {
   dist: {
     overwrite: true,
-    src: ['dist/*.html'],
+    src: ['dist/*.html', 'dist/*.txt'],
     replacements: [
       {
         from: '[[',