add basics concept

[oxe-i]

Adds the first concept to the track.

1. All concepts and their exercises will be first written to a reference folder. Once there are enough concepts, they will be copied to the correct folders before setting the concept exercises key in config.json to true. This is to avoid the concepts showing up to students in the site before they are ready.
2. The generator script was updated to handle concept exercises. The docs were also updated accordingly.
3. Concept exercises' generators are added to the correct folder. This is safe because there is no corresponding folder under either concept or practice exercises (since the concept exercises are still in reference) and so nothing is written (the generator script just logs a fail).
4. I updated bin/update-from-templates to be the main path to propagate any change to files in templates to every concept and practice exercises. The generator script can then delegate this job to bin/update-from-templates.