Addresses regression with jenkins setup that does not use the multiproject setup

The Jenkins Gitlab Hook Plugin allows for two kinds of setups.  One where a Jenkins job is created for each branch and one where all branches are built in a single Jenkins job.  MR !9 added support for the multi-job setup but broke the single job setup.  This request addresses #30 and fixes the regression.

See merge request !27

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Document known issue with Reply by email and multiple application servers.

See merge request !1499

Closes #2867

Bump Ruby and Golang versions

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See merge request !1467

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Closes #2692

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Fix bug where projects would appear to be stuck in the forked import state

A race condition existed between when Rails committed the `import_status` to
`started` and when the Sidekiq worker forked a project. If this fork were quick,
it's possible that the worker would attempt to move into the `finished` state
before the `started` state took effect.

As mentioned in https://github.com/mperham/sidekiq/wiki/Problems-and-Troubleshooting#cannot-find-modelname-with-id12345,
we can either delay the worker to ensure the DB has a chance to update, or use the nice `after_commit_queue` gem to schedule the task after the state machine commit. See:

* https://github.com/pluginaweek/state_machine/issues/191 
* https://github.com/shellycloud/after_commit_queue


Closes #2736

See merge request !1434

Fix Slack notification URL and remove the usage of Ci::RoutesHelper

Fix for #2644

See merge request !1391

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Make instructions for CI disable checkbox clear.

Fix confusion in https://gitlab.com/gitlab-org/gitlab-ce/issues/2663

See merge request !1431

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Add missing path change back

Get CI token from ci projects

This fixes problem when GitLabCiService had different token than stored
in Ci::Project.

See merge request !1425

Update mailroom to 0.5.2

Fixes #2698 and #2711.

See merge request !1424

This allows us to see a breakdown of API requests in New Relic.

Add Troubleshooting section to 8.0 docs

[ci skip]

See merge request !1416