Make em the default tag for italic

[Tug]

Fix

Gutenberg has <em> defined as the tag for italic formatting. Aztec can handle <em> but converts it to <i> when sending back the html to gutenberg-mobile. This causes inconsistencies in the UI and should be addressed.

Test

1. Checkout gutenberg-mobile PR Use format-library for the formatting bar gutenberg-mobile#275
2. Use the following patch to make changes to Aztec on gutenberg-mobile:

curl "https://gist.githubusercontent.com/Tug/50f0a51b7833a94d5f05f4fa7020a289/raw/3317f9ab4dd096cb2a358c44bb5e1085a8a15550/debug-with-aztec.patch" | git apply -v

2. Run the app and check that italic formatting is selected when the cursor/selection is on italic
   formatting

Review

@hypest

[hypest]

Updated the PR, expanded it to make <em> be the default for italic and recused the tests to test for such. Ready for a review.

👋 @daniloercoli , you think you can make a review pass on this one? Thanks!

[daniloercoli]

Nice! With tests updated this PR can be 🚢ed!

Feel free to merge once the conflicts are resolved.

[hypest]

Thanks for the review @daniloercoli !

Let's hold off merging this until the parent PR is also ready to merge wordpress-mobile/gutenberg-mobile#275. This way we can avoid a potential revert in case the parent PR doesn't get merged.

[hypest]

We're giving the "go ahead with merging" for the Gutenberg and gutenberg-mobile side PRs so, merging this.