Rewrite reference processing code in preparation for opening/closing from comment references (#8261)
* 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
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