Move Markdown/reference logic from `Gitlab::Markdown` to `Banzai`
- Moves from `Gitlab::Markdown` to `Banzai`
- Moves filters and pipelines into their own namespace: `Banzai::Filter` and `Banzai::Pipeline`
- No more `autoload`!
- Split up `Gitlab::ReferenceExtractor` into `Banzai::ReferenceExtractor` and `Gitlab::ReferenceExtractor`
- Replace `something(load_lazy_references: true)` by `Gitlab::ReferenceExtractor.lazily { something }`
Goes from:
```ruby
def referenced_merge_requests
references = [self, *notes].flat_map do |note|
note.all_references(load_lazy_references: false).merge_requests
end.uniq!
Gitlab::Markdown::ReferenceFilter::LazyReference.load(references).uniq.sort_by(&:iid)
end
```
to
```ruby
def referenced_merge_requests
Gitlab::ReferenceExtractor.lazily do
[self, *notes].flat_map do |note|
note.all_references.merge_requests
end
end.sort_by(&:iid)
end
```
See merge request !2027
5ffed49fef