Sync: Implement term taxonomy blacklist

[tyxla]

Because of conflicts with filters, we'd like to stop syncing some taxonomies. To take care of that, this PR introduces a term taxonomy blacklist, similar to the one we have for post types. It also

Changes proposed in this Pull Request:

• Define taxonomy blacklist with possible get_* filter conflicts
• Implement the taxonomy blacklist setting
• Filter out blacklisted taxonomies when syncing terms
• Exclude blacklisted taxonomies from term histogram
• Add taxonomy blacklist tests

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• Related with p5TWut-hM-p2 and p7rcWF-12N-p2

Testing instructions:

• Checkout this branch.
• Register a forbidden custom taxonomy (for example bloginfo_rss).
• Create a term in that taxonomy, and verify it doesn't incrementally sync.
• Trigger full sync, and verify it doesn't sync forbidden taxonomy terms.
• Make sure tests pass.

Proposed changelog entry for your changes:

• Sync: Implement term taxonomy blacklist

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: August 6, 2019.
Scheduled code freeze: July 30, 2019

Generated by 🚫 dangerJS against c09c1bd

[lezama]

:magnifique:

[lezama]

Tested thoughtfully, everything seems to work as expected, thanks! 👍