Support patch bundling

[lu-zero]

Patchwork allows to bundle together a list of patches.

• [ ] Add to the model the concept of Bundle
• [ ] Implement a bulk action add to bundle for the patch lists
• [ ] Implement a bulk action remove from bundle for the bundle view
• [ ] List the bundles as it is done for the tags in the patch view
• [ ] Provide patch navigation as implemented for the series

[ftomassetti]

Is this a feature really critical?