[Gutenberg] Use JS bundle from JitPack

[hypest]

This PR removes the dependency on Travis+AWS to get the Gutenberg JS bundle and instead, expects the file to be available via the bridge aar as an assets resource and resources merging.

To test:

1. Build and run the wpandroid app having the wp.BUILD_GUTENBERG_FROM_SOURCE gradle flag to true so the build will depend on the prebuilt JitPack library artifacts. The app should compile and run fine.
2. Run the app, enable GB in the App Settings and open a GB post. It should work as normal.
3. Check out the <rootfolder>/WordPress/src/main/assets folder and make sure there is no index.android.bundle file present. That used to be the JS bundle loaded by GB but now that lives inside the bridge library aar.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[daniloercoli]

Tested and works fine! Both with wp.BUILD_GUTENBERG_FROM_SOURCE gradle flag set to true, and false.

[hypest]

Thanks for the review @daniloercoli !

I added a commit (1437901) to have gradle fail the build if the legacy JS bundle is still present, so we are sure the proper bundle is used. Would you mind doing another pass to include this in the review? Thanks!

To test:

1. touch WordPress/src/main/index.android.bundle to create the legacy file if not already present in your file tree
2. ./gradlew assembleWasabiDebug to build the project. It should fail with an error, otherwise it shouldn't fail if you delete the file.

[daniloercoli]

Works as expected @hypest - LGTM!!
Error: Legacy Gutenberg JS bundle file detected. Please delete it: <root>/WordPress-Android/WordPress/src/main/assets/index.android.bundle. Expression: this.file(legacyGBBundleFile).exists() Open File

[hypest]

Awesome, thanks @daniloercoli !

I'll wait for the LGTM and merge of wordpress-mobile/gutenberg-mobile#526 and then update and merge this one too.