Plugins

This issue evolved, latest state is in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17003

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Companies want to add their own features to GitLab without sharing the code with others by contributing it to the main repository (for security reasons, to iterate quickly, to have something specific to them, or to do something that doesn't meet our definition of done). Right now they have to run on a fork, that makes GitLab hard to keep up to date and we see that they tend not to upgrade.

The proposal is to:

• Rename project services to plugins (to make it clear that we have plugins and because project services never was a good name)
• Allow local plugins by loading them automatically if they are in a directory that is not under version control in /var/opt/gitlab/gitlab-rails/plugins