Turn reference regex constants into methods
`Mentionable::ReferenceRegexes` used to define the following two constants: 1. DEFAULT_PATTERN 2. EXTERNAL_PATTERN These two constants were built using some of the class methods that reside in this same module. In EE we redefine one of these methods by using `prepend` at the start of the `ReferenceRegexes` module. This poses a problem: we can not move the `prepend` to the end of the file, because the constants later on depend on it. To resolve this problem, this commit turns these constants into class methods that memoize their results. This allows EE to redefine the appropriate methods before these two class methods are used, in turn allowing us to move the `prepend` to the end of the file. See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8198 for more information.
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