Highlight new layers

[crisner]

Fixes #427 (<=== Add issue number here)

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[crisner]

Screenshot of the feature:

Index page:

Onelinercode example page:

[jywarren]

This looks great! Maybe sometime we'd make them fade away, but this is awesome as-is too. Thanks!!!

[jywarren]

Just needs a rebase! Thank you!!!!

[crisner]

@jywarren and @sagarpreet-chadha, I have fixed some inconsistencies in my latest commits here. This PR is ready to be merged after the commits are reviewed. Thanks!

[crisner]

Fixed layer highlight working only on one map because of conflict in layer ids.

[jywarren]

Awesome work here!!!! Merging!