Set label priority

Description

Right now there is no good way to prioritise between issues. Prioritizing can be important with high volume, seemingly similar issues.

Proposed implementation

• A project member, master, should be able to prioritise certain labels. Not every label needs a priority
• Prioritization happens by ordering a list of prioritised labels. Example here, but other ways acceptable as well: https://gitlab.com/gitlab-org/gitlab-ce/issues/14189#note_4200885

By not having medium, high priorities and by making it optional, people are free to opt-in to priorities. It also gives you the power to automatically make certain things a high priority, like security. Additionally, this system still allows you to create a low, medium, high priority system if you so please.

We should be able to:

• sort issues by priority in the issue list
• by default sort issues by priority in the milestone view
• sort Todos by issue priority

How priority should work

Priority is just the order of labels. The first has the highest, the last the lowest.

For issues with multiple labels, you can use this logic for sorting:

• issue with the highest priority label always on top
• if multiple with same highest label, issue with single highest label and most other labels

We don't want to score them based on their priority labels, but that would bias issues with many labels with a low priority.

Proposed UX

As discussed with @jschatz1:

• priority labels on top
• fixed top, only bottom paginates
• button next to every label to bring it up to the prioritzation

cc @DouweM @dzaporozhets @rspeicher @stanhu

Thanks @DouweM for the idea!