Email Markup, Templating, and Formatting Tools

This codebase currently contains one re-usable email ([the "thanks for signing up for our emails" email](https://github.com/hacksu/khe-revengeance/blob/dev-main/server/emails/not-inlined/welcome_not_inlined.html)); it is written in normal CSS and HTML, which then needs to be transformed somewhat so it works in email clients that generally support a subset of CSS and HTML. The transformed version is saved in a second [email HTML file](https://github.com/hacksu/khe-revengeance/blob/dev-main/server/emails/welcome.html).

However, manually taking HTML and running it through a CSS inlining tool like I did is like, slow. We would be better off writing emails with something like [mjml](https://github.com/mjmlio/mjml) (an older, mature emailing markup language) or [react-email](https://github.com/resendlabs/react-email) (a newer, very popular set of React components that can be easily server-side rendered) or [vue-email](https://github.com/Dave136/vue-email), so they can be automatically translated into email-compatible CSS and HTML. Using either of those last two would also give us templating abilities.

Template layouts with headers/footers/backgrounds could then be created and used for whatever emails are written in the email composer component by staff.