Update project template to rails 6

What does this MR do?

This project template is severely outdated and broken #31 (closed), blocking our QA pipelines (internal link). This updates it to Rails 6, and renames the application module from Demo to MyApplication.

Steps taken to generate

# clean up
rm -rf *
rm -rf .bundle .ruby-version

# regenerate
rails new my_application --database=postgresql

# move from nested folder
rm -rf my_application/.git
mv my_application/* .
mv my_application/.* .
rm -rf my_application

# get the old readme back
git checkout -f README.md

# initialize the application so it's runnable
bin/rails db:create
bin/rails db:migrate

# MANUALLY: added rexml to Gemfile
bundle

# add linux to lockfile
bundle lock --add-platform x86_64-linux

# re-add a welcome page
bin/rails generate controller Welcome index --skip-routes
# MANUALLY: Edited config/routes.rb and app/views/welcome/index.html.erb

Fixes #31 (closed)