Improvements to sequence tutorial. ...

All the stuff that I didn't have time to due during the semester:

* Fixed some minor inconsistencies between diagrams.
* Added some code examples where appropriate.
* Added section on data fields.

Add alpha to diagram backgrounds to dedazzle them in the dark theme

Embedded diagrams in sequence tutorial. Also split out the sections.

General clean up. Embed images. Add icon macro.

Improved merge diagram

Complete and release lab 3