copy r13239 awesome_nested_set modification to Issue model

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branch : ../sandbox/rails-4.1
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/sandbox/rails-4.1%4013290
revert r13239 awesome_nested_set modification

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branch : ../sandbox/rails-4.1
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/sandbox/rails-4.1%4013291
Gemfile: use awesome_nested_set gem

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branch : ../sandbox/rails-4.1
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/sandbox/rails-4.1%4013292
mysql gem no longer used.

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branch : ../sandbox/rails-4.1
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/sandbox/rails-4.1%4013293
Reverts r13279.

uninitialized constant Rake::TestTask error: bundle is not yet completed when running rake -f lib/tasks/ci.rake config/database.yml on the CI server. What was the problem with Rake::Task["test"].invoke ?

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branch : ../sandbox/rails-4.1
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/sandbox/rails-4.1%4013294
NameError: uninitialized constant ActiveSupport::TestCase::Shoulda error on the CI server.

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branch : ../sandbox/rails-4.1
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/sandbox/rails-4.1%4013295
Add missing fixtures.

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branch : ../sandbox/rails-4.1
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/sandbox/rails-4.1%4013296
Removed deprecated :action option in #link_to_project.

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branch : ../sandbox/rails-4.1
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/sandbox/rails-4.1%4013297
Don't change the date format for the other tests.

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branch : ../sandbox/rails-4.1
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/sandbox/rails-4.1%4013298