Unify bugzilla and github tags/labels

Currently, we have different tags and labels on github and bugzilla which makes find all the bugs related to one label difficult. I propose we have the same tags and do some clean up while we’re at it.

In bugzilla we’ve usually done pdfjs-c/d/f-name where c=category, d=detail, f=flag. I’ve never found the category useful. Only the part in the detail or flag has helped me find bugs, so I think we should get rid of the letters and only have the part after d/f. e.g. [pdfjs-d-blend-mode] becomes [pdfjs-blend-mode].

For github, we have the number before the label. Similar to the category on bugzilla, I don’t usually find the number helpful (I don’t even really remember what all the numbers mean). I propose we drop the numbers and just use more detailed ones that match up with bugzilla.

[anantraghuvanshi]

@brendandahl In such cases, we could use tags/labels which are specific to certain other problems and which depicts some meaning to the reader, like help-wanted, enhancements, bugs, question, or problems. Along with that, if we consider extra precision, then we could add something like difficulty or understanding level required, which could be basic, easy, medium, hard, or expert.

[brendandahl]

Alright, numbers removed. Whiteboard tags in https://docs.google.com/spreadsheets/d/1_7foFdAoR4lu64qJQe9k4zODTIUxkGstcrO6LKZ0WqU/edit#gid=0