[Demo template] Add standardized alert for lack of feature support

[melanierichards]

What this PR does

What it says on the tin :P The actual JS functions adding this alert break the code style, but this is so that they work in the older browser versions they're supposed to be targeting.

Requirements

• [x] My PR follows all applicable accessibility requirements (See .github/ACCESSIBILITY_REQS.md).
• [x] My PR follows the CSS code style guidelines (See .github/CSS_STYLE_REQS.md).
• [x] I have linted my code using npm run lint:css -- demoDirectoryName/**/*.css and npm run lint:js -- demoDirectoryName/**/*.js, and have fixed the errors.

[molant]

@melanierichards there are conflicts in this branch, otherwise it LGTM

[melanierichards]

@molant great, thanks!