Refactor labels on all repositories

[dkastl]

I think the labels on all repositories need some review:

• there are so many, that for regular users it's difficult to pick the right one(s)
• naming could be improved to make them more consistent, maybe adding a prefix to apply some sort of schema, if there are many
• coloring could be more consistent
• general ones should follow common patterns in all repositories

[dkastl]

Example: https://github.com/pgRouting/admin/labels

We could prepend ISSUE > ... to general issue labels for example.
This makes sure that they are in a less chaotic order, because labels are ordered by alphabet.