SEO: do not display any tags in post titles display in meta tags

[jeherve]

Fixes 4478-gh-jpop-issues

Changes proposed in this Pull Request:

Some plugins use the the_title filter to add HTML to post titles (to add schema.org metadata for example).
We consequently cannot assume anything returned by get_the_title() will be safe to display in a meta tag.
Let's use the_title_attribute() instead to avoid having any unwanted HTML.

Testing instructions:

• Start from a site running Jetpack Premium and where you've set posts to use a custom meta tag in SEO settings in Calypso:
  

• Install the Easy Digital Downloads plugin

• Create a Download

• View it, and check the meta tag.

  • Without this patch, you will see HTML like so:

<title>&lt;span itemprop="name"&gt;A test download&lt;/span&gt; and Testing Jetpack and Testing Jetpack</title>

• With the patch, the HTML disappears.

Proposed changelog entry for your changes:

• SEO Tools: do not display any tags in post titles display in meta tags.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello jeherve! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D35160-code before merging this PR. Thank you!

[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: December 3, 2019.
Scheduled code freeze: November 26, 2019

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