Type cleanup & standardization work

[mmmavis]

Closes #2806

• We'll need to have a follow-up to clean up font related CSS rules in individual .scss files. I tried to do it as much as I could but there are just too many old, unused files sitting in the codebase which made my cleanup work challenging. Once this PR is merged and those unused files are removed, we should have a follow-up PR to finish the CSS rules cleanup.
• Buyer's Guide styling still need to be cleaned up (outside of the scope of this ticket/PR)

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Reminder

Since new stylings and changes have been introduced in the new heading hierarchy system (e.g., <h1> <h2> <h3> <h4> <h5> now have new stylings), right after we push this PR to production we'll have to go through CMS pages and make changes to heading type to make sure headings are in the right style we want. e.g., change <h3> elements in CMS to <h2>

We need to make sure we have time blocked for updating things in CMS pages before we push this live

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Changes made to h1-h4

1. old h1 default stylings have been updated
2. old h2 styling set becomes the new h4 styling set
3. old h3 styling set becomes the new h2 styling set
4. old h4 styling set becomes the new h3 styling set

For details see MozillaFoundation/Design#319 (comment)

[mmmavis]

This heading mixin makes sure all headings will have all three rules set to avoid unintentional styling inheritance.

[mmmavis]

@Pomax @youriwims

This is a HUGE PR with a ton of tedious changes so I don't expect you both to go through every single code changes. I think it would be enough if you can just focus on changes made to .scss files as @sabrinang and I have already gone through few rounds of peer design-code review.

If you wanna learn more about the ongoing brand work, @sabrinang has things documented here https://foundation.mozilla.org/en/docs/brand/design/Typography/ (type section is pretty much up to date)

[mmmavis]

Oh the typography section on style guide has been updated too (in case you'd like to an overview list )

[youriwims]

✨ 👏 👍 💫 🙌 💕 😌 ⭐ all the emojis ha—great job on this type work guys @mmmavis @sabrinang!