Allow for translating strings in Learning Mode templates

[jom]

Is your feature request related to a problem? Please describe

Right now, our Learning Mode templates have english default strings that aren't translated. They are editable, but it would be nice if they could be initially translated. An example string is Contact Teacher.

Describe the solution you'd like

We should be able to convert them to PHP files and in Sensei_Course_Theme_Templates::get_block_templates() include the files instead of use file_get_contents.

Describe alternatives you've considered

Forcing the user to manually translate the text in the site editor.