Allow distinguishing groups of labels via text means

Labels are grouped only by color, e.g. all priorities are blue. Labels are typically sorted in lexical order, e.g. when interacting with the issues search/filter box. So it's not easy at the moment to list all labels of type "category" or "priority".

My current proposal is to prefix labels like this: "P: $priority", "C: $category", or "Priority: $priority", "Category: $category", etc.; that's a common pattern on GitHub/GitLab. It plays well with the aforementioned lexical sorting and it's better than color-only grouping, for a11y purposes.

I suspect other Tails folks will have their own ideas on how exactly the labels should be called, but I believe something like "Category: $category" would be a better starting point for discussing this than the current, color-only implementation.