Introduce templates for new packages

[hellcp-work]

Introduces a simple template support to new project creation. Currently only supports one type of a package, SUSE style RPM, but it can be easily expanded to support everything that may be needed.

To verify:

1. Create a new project for templates
2. Set the OBS:PackageTemplates attribute on the project
3. Create a package in the template project
4. Create a file with name like: {{ package.name }}.extension.liquid and contents like {{ user.name }} (other options are in src/api/app/services/package_service/templater/{project,package,user}_drop.rb files)
5. Go to any other project
6. Click a create new package button
7. Write metadata about the new package
8. Select the template you created in the template project
9. Observe the package be initialized with a set of files

TODO:

• Create test data
• Create tests
• Write documentation for the feature

[hellcp-work]

Explore new workflows multi file upload enables

[obs-bot]

Review app will appear here: http://obs-reviewlab.opensuse.org/hellcp-work-templates

[hennevogel]

This would require code changes and deployments for every template change. I'm not sure this is such a good idea.

[hellcp-work]

This would require code changes and deployments for every template change. I'm not sure this is such a good idea.

I went for a similar mechanism to image templates now. I implemented basic templating with liquid, in order to prefill some metadata, to avoid having to do that manually