Update HTML files to use new design language styles

[jgray7019]

Closes #1099

This PR updates all styles and HTML used in the html.erb files to adhere to the new design language that has shipped across the store admin.
Design Language Figma for reference.

Tophatting

1. You will need to set up a test app using the shopify_app gem.
2. Checkout this branch of the gem. Re-start your server in the repo for your test app.
3. Run the local version of your app in any browser (bundle exec rails server) to view the install app page (new.html.erb). This page was not totally following Polaris styles before so I have updated the HTML and styles to better follow Polaris guidelines.

4. Using Safari, install your app on a development store to be redirected to the top_level_interaction.html.erb page.

5. Click Continue and you will see the request_storage_access.html.erb page.

6. Change you Safari version to 12.0.0 to see the enable_cookies.html.erb page. You can do this by clicking on Develop > User Agent > Other

And then changing the version to `12.0.0`

This should now show you the correct page:

[paulomarg]

🎩 ed, and from the code / app perspective it seems good to me!

In any case, I'm re-adding the team review if someone wants to take a closer look at the style itself, but as far as I can tell the app itself is working just fine.

[tiffanytse]

Im not sure how much of this is just auto generated and pulled in; or what you've written yourself. Just left a few comments. Looks good otherwise!