* Move to goldmark

Markdown rendering moved from blackfriday to the goldmark.

Multiple subtle changes required to the goldmark extensions to keep
current rendering and defaults.

Can go further with goldmark linkify and have this work within markdown
rendering making the link processor unnecessary.

Need to think about how to go about allowing extensions - at present it
seems that these would be hard to do without recompilation.

* linter fixes

Co-authored-by: Lauris BH <lauris@nix.lv>

* Add support for local vs. remote xrefs

* Add doc for references

* Docs: fix cases not currently supported

* One more doc fix

* Doc: mentions for teams and orgs

* Change !num ref concept, no change in functionality

* Fix test

* Improve table of issue reference types

* Fix paragraph mark

* Add close/reopen from comment functionality

* Fix comment

* Rewrite closing/reopening template

* Check xref permissions, move action to services/pull

* Fix RefIsPull field

* Add xref tests

* Fix xref unique filter

* Only highlight keywords for actionable xrefs

* Fix xref neuter filter

* Fix check return status

* Restart CI

* Enable punctuations ending mentions

* Improve tests

* Add settings for CloseKeywords and ReopenKeywords

* Fix and improve tests

* Use sync.Once() for initialization

* Fix unintended exported function

* Add a markdown stripper for mentions and xrefs

* Improve comments

* Small code simplification

* Move reference code to modules/references

* Fix typo

* Make MarkdownStripper return [][]byte

* Implement preliminary keywords parsing

* Add FIXME comment

* Fix comment

* make fmt

* Fix permissions check

* Fix text assumptions

* Fix imports

* Fix lint, fmt

* Fix unused import

* Add missing export comment

* Bypass revive on implemented interface

* Move mdstripper into its own package

* Support alphanumeric patterns

* Refactor FindAllMentions

* Move mentions test to references

* Parse mentions from reference package

* Refactor code to implement renderizable references

* Fix typo

* Move patterns and tests to the references package

* Fix nil reference

* Preliminary rendering attempt of closing keywords

* Normalize names, comments, general tidy-up

* Add CSS style for action keywords

* Fix permission for admin and owner

* Fix golangci-lint

* Fix golangci-lint