[Gutenberg] Adding Tracks properties to gutenberg enable/disable event

[etoledom]

This PR is the WPiOS side of: wordpress-mobile/WordPress-Android#10300

Add additional properties to gutenberg_enabled , and gutenberg_disabled analytics events:
The property source can now have the following values:

• via-site-settings
• on-block-post-open
• on-site-creation

Disclosure:
With this changes, are not tracking the switch event in cases where:

• We are setting Gutenberg/Aztec as part of a migration
• We are setting Gutenberg/Aztec as part of sync with remote
• We are setting Aztec as the default value

Is this the proper behavior? cc @daniloercoli @hypest

To test:

Test 1:

• Flip the block editor setting in Site settings and make sure the property is added correctly

Test 2:

• Create a new site, and make sure the property is added with the correct value
• Tap on new post, and make sure nothing is tracked

Test 3:

• Clear your editor settings on the web
• Re-install the app
• open a block based post
• the popup should appear, and the correct property value bumped with analytics

Update release notes:

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