Add wx prefix to custom classes

[partly-igor]

What does this PR do? 🛠️

This PR adds a wx prefix to the custom utility classes and components we use. It add to the work started in #1313 and
together with that PR closes #1247

What does the reviewer need to know? 🤔

Two notes:

1. This PR is on top of Reorganize SASS files and classes #1313 and should be reviewed/merged second (or alternatively just review this one, though there are more files changed so the diffs for the prefix addition might be longer)
2. Seeing in in practice here, it's a good moment to re-evaluate if we do want to go ahead with this approach of using the prefix for all custom classes.

[greg-does-weather]

This looks good to me! I made some suggested edits related to PHP test names, but it's not breaking or blocking either way.

[partly-igor]

Thanks for catching those