Upgrade font-awesome from 4.7

[jcoyne]

We're currently using font-awesome 4.7 which was released over 7 years ago: https://www.npmjs.com/package/font-awesome The current version is 6.5 (https://docs.fontawesome.com/web/use-with/scss/)

4.7 uses the / operator which is currently deprecated in sass:

12:50:39 css.1  | Recommendation: math.div(30em, 14) or calc(30em / 14)
12:50:39 css.1  |
12:50:39 css.1  | More info and automated migrator: https://sass-lang.com/d/slash-div
12:50:39 css.1  |
12:50:39 css.1  |    ╷
12:50:39 css.1  | 12 │ $fa-li-width:         (30em / 14) !default;
12:50:39 css.1  |    │                        ^^^^^^^^^
12:50:39 css.1  |    ╵
12:50:39 css.1  |     node_modules/font-awesome/scss/_variables.scss 12:24  @import
12:50:39 css.1  |     font-awesome/scss/font-awesome.scss 6:9               @import
12:50:39 css.1  |     app/assets/stylesheets/application.sass.scss 11:9     root stylesheet