Update email sent to new users

[NEStock]

EMBER-ify email

[yarikoptic]

I love git grep to locate myself some code to change ;) e.g. I would have gone through those

❯ git grep -l dandiarchive.org -- web dandiapi
dandiapi/api/templates/api/account/questionnaire_form.html
dandiapi/api/templates/api/mail/approved_user_message.txt
dandiapi/api/templates/api/mail/registered_message.txt
dandiapi/api/templates/api/mail/rejected_user_message.txt
dandiapi/api/templates/api/root_content.html
dandiapi/settings.py
web/README.md
web/src/components/FileBrowser/FileUploadInstructions.vue
web/src/components/UserStatusBanner.vue
web/src/utils/constants.ts
web/src/views/CreateDandisetView/CreateDandisetView.vue
web/src/views/DandisetLandingView/DandisetLandingView.vue
web/src/views/DandisetLandingView/DownloadDialog.vue
web/src/views/DandisetLandingView/ExternalDandisetServicesDialog.vue

and while at it, annotated with a comment TODO: vendor or alike to ease later defining what to work on to extract to be configured per instance. (there still might be some places where it is ok to leave pointing to dandiarchive.org)