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[mike-ward]

Description

• Add Dropdown that shows links to archived versions of the documentation.
• Modify generate.js to checkout /archive from `gh-pages', read folder for previous version names and then reset and delete.
• Add rimraf as dev dependency to delete checked-out archive folder.
• Change layout.html to provide replace target for generated <select> tag.

Motivation and Context

#2334

How Has This Been Tested?

Presented working mock-up on Gitter chat for approval.
Ran locally after running gendocs

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated docs/change-log.md

[dead-claudia]

@tivac I requested a review from you because you're most familiar with how Travis is set up. This was based on my guidance, and I'm just looking for a sanity check here in case I screwed up on my end.

[StephanHoyer]

@mike-ward I just rebased this PR to current next branch. Maybe you can give it a review?

[StephanHoyer]

just rebased against next. Had to rework some stuff.