Add staging of Online Editor tags and PRs

[Kr0nox]

This PR adds a workflow that builds the online editor and deploys that version in a subfolder on the https://github.com/DataFlowAnalysis/OnlineEditorStaging repo. They can be accessed under https://dataflowanalysis.github.io/OnlineEditorStaging/

There are 3 ways of deploying:

• Every time a tag is pushed it gets deployed
• When somebody comments /stage on a pr
• Run manually on a branch through the actions tab

[Nicolas-Boltz]

Looks good to me ;)