Refactor service external issue tracker and wiki

What does this MR do?

Follow-up from !45781 (merged).

Small refactor:

• Change issue_tracker? to external_issue_tracker? method.
• Add external_wiki? method.

The methods are aligned with the Service scopes in L61 and L62.

scope :external_issue_trackers, -> { where(category: 'issue_tracker').active }
scope :external_wikis, -> { where(type: 'ExternalWikiService').active }