Update the Sensei Lesson Template

In the process of digging into a different issue, @imran92 discovered the following:

While on it, a thing I’ve noticed is Learning Mode is using a slightly edited older version of the Lesson (Learning Mode - Default) template. So it’s missing the new improvement we’ve made in that template. For example, we’re not using the default "Lesson Actions" instead of the learning mode specific "Lesson Actions (Learning Mode)" block in the bottom of that template.

Some benefits of this block are that it allows changing the Text color, Fill color, border color and width, padding etc of all the Lesson action buttons (like View Quiz, Complete Lesson) etc. We don't need to update the whole template, but I think it can be nice to replace the current Lesson Actions block with the other one for these improvements.

I'm moving this conversation here so we can test this upgrade, in order to roll it out to Learn.WordPress.org sitewide

Start with the Learning Mode - Default template and compare its current Lesson Actions (Learning Mode) block with the default Lesson Actions block. Replace the current block as described, then verify that the lesson action buttons retain the expected behavior and that the template can be tested for a sitewide Learn.WordPress.org rollout.