Generic links to resources in material

[fcooper8472]

Some course material contains code blocks with explicit links to download material, e.g.:

wget https://www.uhpc-training.co.uk/material/software_architecture_and_design/procedural/inflammation/inflammation.zip

in technology_and_tooling/testing/automated_testing.

This causes a problem, as one group might use uhpc-training.co.uk and another uses train.oxrse.uk. This means there's a tug of war between different users to update the material to their own links.

We need the material to be able to contain something like:

wget {{{ base_url }}}/material/software_architecture_and_design/procedural/inflammation/inflammation.zip

and for Gutenberg to replace that with the appropriate base.