Support for multiple labels

[i5hi]

Current the labeling system only supports a single label per element.

This leads to issues with label inheritance.

Currently labels are inherited only if the child does not already have a label. Instead, the child should always inherit the label and retain any existing labels.

This requires allowing a single element to have multiple labels.

• [ ] No need to allow label edits, just create and delete
• [ ] Display labels like github issue tabs
• [ ] For BIP329, use comma separated list for items with multiple labels