Pre-select github repo and ref

[oliverroick]

Contributes to #66: Pre-select GitHub repo and GitHub branch when URL params are set

• The heavy lifting is done in state.js where we check for the presence of the URL parameters (binderProvider, binderRepo, ref), and set the selected profile to the one that supports dynamic image building. If more than one profile support dynamic image building, we show a warning and don't set the form fields.
• Validation of the Github repo string is still done in the back, but automatically executed when there is a default value (ie. the value from the binderRepo parameter.

[batpad]

@oliverroick sweet! I tested this by running locally and a URL like http://localhost:8000/hub/spawn?binderProvider=gh&binderRepo=nasa-impact/pangeo-notebook-veda-image&ref=main and all the fields were filled in correctly and it auto-selected the Build Your Own Image option.

Since I don't think this is going to block any other work and @yuvipanda is back in a couple days, does it feel okay to leave this PR open until @yuvipanda is back? Am fine merging as well if there's other stuff you feel you want to do on top of this.

This looks really good, thanks again!