Add support for gameLevels query parameter

[arouinfar]

Recently, a user expressed their concern that the isotope symbol notation could be confusing if students are comparing it to the typical periodic table notation (see #229). Our notation choices in the sim are correct, but could be confusing to younger students. The Symbol screen is easily eliminated with screens, but game levels 3 & 4 still contain the symbol notation.

In fourier-making-waves, we used the gameLevels query parameter to allow users to select which levels are available on the level selection screen. I think we should do the same here the next time the sim is republished from master.

[arouinfar]

Also note that challengesPerLevel doesn't appear to work in the published version of the sim, so let's review that too when we get to this issue.

For example:
https://phet.colorado.edu/sims/html/build-an-atom/latest/build-an-atom_en.html?screens=3&challengesPerLevel=1