Remove overlay module

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sub-issue from #7

[RobLoach]

[sreynen]

Looks like this is done already in #26 ?

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There's a pull request at backdrop/backdrop#26, but it's not passing tests.

Issue 26 was a placeholder for the pull request with the same number. We separated the issue queue and the main repository because of Github's permission limitations. Sorry it's confusing. It won't happen any more. :)

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New pull request at backdrop/backdrop#54 solves all the tests except for one last failure.

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There's another PR attempt at backdrop/backdrop#59. Though it has the same single failure.

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@michellemurrain's PR at backdrop/backdrop#59 is passing tests! Seems like it may just need some review. :)

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This has been fixed in PR backdrop/backdrop#83. Thanks to @michellemurrain for the help on this one!