Update Plugin schema

Plugin.tags (many-many with Tag) is now Plugin.keywords (array).
Refactor dummy data generation on bootstrap
Add plugin and version management code

Creation via POST and request to npm registry to fetch package metadata.
Add plugin update controller
Add destroy method for plugins
Merge branch 'feature/plugin-management' into develop
Change single line `if` and minor fixes
Change single line `if` an other code refactoring

Replaced all error blocks with the new ErrorManager service.