Project#has_external_wiki? can write to the database in a GET request

As seen in https://gitlab.com/gitlab-org/gitlab-ee/issues/1217#note_19302371

Before caching this value, we should check whether we can write to the database, either using Gitlab::Geo.secondary? (EE) or following the implementation of https://gitlab.com/gitlab-org/gitlab-ce/issues/37534

/assignee @to1ne