chore: set up mjml for building email templates

internal/mail/.mjmlconfig

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+{
+  "options": {
+    "minifyOptions": {
+      "minifyCSS": true
+    }
+  },
+  "packages": []
+}

internal/mail/Makefile

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+MJML=npx mjml
+MJML_FILES=$(wildcard mjml/*.mjml)
+HTML_DIR=html
+
+build:
+	$(foreach file, $(MJML_FILES), $(MJML) $(file) --config.minify --config.useMjmlConfigOptions -o $(HTML_DIR)/$(basename $(notdir $(file))).html;)
+
+.PHONY=build

internal/mail/README.md

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+## Email templates
+
+These email templates are generated using [MJML](https://mjml.io/). It may be
+helpful to add an [IDE extension](https://documentation.mjml.io/#development) to
+aid in development.
+
+## Build templates
+
+All templates in the `mjml` directory can be built by running the following command:
+
+```
+make build
+```