Comm panel email templates

[willgearty]

Lots of chapters use email templates in various ways, such as:

• Keeping a local text template, filling it in offline, then copying and pasting into the comm panel (I did this when I was at Stanford Splash)
• Pulling up an old email in the comm panel, editing it, then sending again
• Custom code to insert the comm panel text into a template (Yale does this)

We should implement email templates in a more official manner. This would likely involve the following incremental steps:

• Pull in the custom changes from Yale's branch to support template for emails. By default, the template would be blank. (see Add default email template #1898)
• Implement a way for chapters to then supply multiple templates (and perhaps seeding this with templates of our own e.g., https://github.com/sendgrid/email-templates)
• Improve on the UI to make the selection of templates more visual like proposed in Comm panel template UI #2885
  (a full-fledged template builder seems out-of-scope, but is perhaps a pipe dream)

[milescalabresi]

First box done by #3817