Security fix: Security check on email template preview page

[timbocode]

Backport from woocommerce/woocommerce@60148d7

See also #214

File affected: includes/admin/class-wc-admin.php

[nylen]

@bahiirwa it's a good idea to explain what was tested/reviewed, we shouldn't just check the "approved" box with no context.

See also #214 (comment) - we should probably make sure we agree on the approach we are using to review and approve these changes.

[timbocode]

Testing

• Manually checked the email preview in admin. All seems OK.

[bahiirwa]

Tested like an ordinary user.

• Admin preview also works fine for me.
• Functionality to add templates to the theme all remain working fine. Nothing is broken.

[nylen]

I compared the changes here against woocommerce/woocommerce@60148d7 and the end result looks good.

The changes themselves are different, because there was another previous commit to this line that we haven't specifically included yet: woocommerce/woocommerce@14d9678

Since this commit also includes escaping changes in other lines I think we should include it in this PR.

[bahiirwa]

Whats the way forward on this PR?

[timbocode]

Awaiting approval from @nylen.

As discussed and agreed with @nylen. An exercise in continuity planning. Reduces single-person dependency.