[28728] Merge OpenProject CE plugins under modules/

[oliverguenther]

• Merges all CE plugins into modules/*, dropping the openproject prefix of all previous plugins.
• Creates a new Gemfile.modules definition that is automatically sourced and lives next to Gemfile.plugins (which can be used by downstream to add to the Gemfile as it's done now).
• Merges the CE .travis.yml to provide full CE testing.
• Adds a conditional branch check to skip plugin builds when branch starts with core/*
• Maintains full history of each plugin by merging with --allow-unrelated-histories from temporary remote

/cc @opf/developers