fix(style): set line-height to improve global body legibility

[karansapolia]

fixes: #6632
@ryanfeeley @lmorchard @shane-tomlinson review please. Hope this addresses Ryan's comment.

[karansapolia]

@lmorchard Any idea why the travis ci build is failing? Shows an advisory about an outdated module https://npmjs.com/advisories/788

[karansapolia]

Improves overall text and elements spacing throughout the website. For example,

Before:

After:

And others. Like the positioning of buttons in the profile avatar change modal. sizing of error messages and the positioning of terms of service and privacy notice links.

[ryanfeeley]

Hmmm… also appears to mess with non-paragraph text like "Not set". This could mess with layouts. We should probably test this pretty extensively before reviewing. As a UX designer, I'm not sure how to proceed.

[karansapolia]

@ryanfeeley I agree. What do you suggest? Pushing just for paragraph text at the moment?

( Is the golden ration spacing guideline applicable for non-text elements? Things feel more evenly spaced out, like in the avatar change modal and some error dialogues. Could be entirely subjective though. )

[karansapolia]

I am not very knowledgeable about UX, so going for more than what was requested does not make a lot of sense here. I am keeping this PR limited to what was originally requested. That is, line-height fixed to 23px for p tags throughout FxA.

@ryanfeeley would request you to review the changes again. Thanks.

[shane-tomlinson]

This repo has been deprecated and migrated to https://github.com/mozill/fxa. Please open this PR against that repo.