Skip loading the Learning Mode styles when Sensei is not available

[gabrielcaires]

Partially solve Review spacings and alignment on the course theme#6834

Changes proposed in this Pull Request:

• Skip the learning mode style when sensei is not available
• Add a temporary hook to skip the loading when Sensei is available and able to provide a new learning mode style.

Testing instructions

• Go to this branch
• Disable the sensei-lms plugin
• Go to the home page
• Check if the learning-mode.css is not loaded
• Enable the sensei-lms plugin
• Check if the learning-mode.css is loaded.

Context

The sensei team is delegating the responsibility to load the learning mode style to the plugin, instead of the handle it on the theme. It will help us to avoid coupling between the theme version and the plugin version.

[donnapep]

Looks good! When do you think we should consider removing the learning-mode.css file altogether? Perhaps with the next major release after the next one? If so, perhaps we could add a note in the file about it so we don't forget.

[gabrielcaires]

@donnapep I created this issue #7429 . I also added as part of our release after the current one.