replace jumpto.js with skipto.js version 4.2

[jongund]

This pull request changes the code base from using JumpTo.js to SkipTo.js.

The 4.2 version of SkipTo.js has been updated to fix the following issues:

1. Removing copyright code concerns by using the skipto.js script instead of the jumpto.js script, see SkipTo copyright statement in code.
2. Selection of main landmark option on the home page, by insuring focus moves to a visible element.
3. The Skip To menu button labeling issue (e..g. "opton" is now pronounced instead of "command" on macOS).
4. The skipto menu of options will now scroll if the number of items exceeds the size of the window to display them all.

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[richnoah]

@jongund we are in the process of getting ready for a coordinated deploy with WAI on the morning of 6/24. We will review this PR again and merge it in. Please refrain from making any other commits so I can make sure all is in good order for the deploy.

[mcking65]

@richnoah Idon't concur with latest changes by Jon; I need to fix the license/copyright to be what is appropriate based on agreement with skipto.

[mcking65]

Thank you all; this is good to ship now. I will merge.