Use Bootstrap v5 and Font Awesome v5

[peterdesmet]

Visible changes:

• Content is slightly wider: https://getbootstrap.com/docs/5.1/migration/#grid-updates
• Links are underlined by default: https://getbootstrap.com/docs/5.1/migration/#content-reboot-etc
• Tables have a new layout: https://getbootstrap.com/docs/5.1/migration/#content-reboot-etc

Important for content:

• If you use font-awesome icons, use the fas or fab rather than fa: https://fontawesome.com/v5.15/how-to-use/on-the-web/referencing-icons/basic-use
• If you use Bootstrap classes, note that -left and -right are renamed to -start and -end. float-left and float-right are still supported in Petridish.

Fixed issues:

• 43

• 19

• 36

• 27

• 16