Improve rendering performance of Banzai
Related issues:
* gitlab-org/gitlab-ce#14315
* gitlab-org/gitlab-ce#13651
TODO:
* [x] `Banzai::Filter::ExternalIssueReferenceFilter#call` calls `Project#default_issues_tracker?` which runs a query on every call. Memoizing this only works if the `Project` instance is shared between instances of this filter. Either way it should at most only run this method once.
* [x] Fix the two failing specs
* [x] Clean of the various `call` methods in the filters where I inlined methods
* [x] Remove the `replace_XXX` methods in ReferenceFilter now that they're no longer used.
See merge request !3389
