Add form submission success page, improve 404 page and more

[salim-b]

• Add custom form success page (only works with Netlify Forms out-of-the-box; Formspree doesn't support this anymore in the free tier, but paying customers can manually set the URL (/message_sent.html) in the Formspree UI)

  Predefined image on success page is public domain; can be customized in config.toml by setting params.contact.form.success_image

• Append lang ID to contact form name for Netlify to ensure proper redirects

• Enable params.contact.form.use_netlify by default since Netlify Forms offers a superior free tier than Formspree.io

• Change Bootstrap's default navbar collapse breakpoint of 768px to 992px (=desktop) since the default menu has too many items to fit on one line between desktop and tablet width

  Style source: https://silvawebdesigns.com/how-to-change-the-bootstrap-navbar-breakpoint/

• Fix wrong media-query order in _404.scss

• Fix 404 (and other custom) page height to be truly 100% viewport height:

  • Ensure header min-height of 90/100px (dependent on screen width)
  • Ensure footer min-height of 205px and center text vertically
  • Assign page min-height of calc(100vh - ([header-height] + [footer-height]))

• Remove duplicated copyright line on 404 page

• Move static/images/marker.png from exampleSite to theme since users most likely won't replace it with custom image

• Fix typo in in CSS class .footer-manu -> .footer-menu and update all references

• Lint some SCSS files

• Update precompiled SCSS files under exampleSite/resources/_gen/