Custom cell tags: `disable-execution`, `auto-execute`, `disable-download`

These tags do the following:

• disable-execution: removes the Thebe rocket, making it clear a notebook shouldn't be run
• auto-execute : automatically runs the notebook, especially useful for exercise pages
• disable-download : removes the ability to download the notebook source code. Note, the source code is still available for download, so smart students can still access it. The only way to circumvent this is to add a step in the build process that removes unwanted source files.
• keep-output: Keep the cell output. Recommended you only use this when you remove the input too.

These tags can be added to any cell, but by convention keep them on the header cell since they affect general notebook behaviour.