includeExternal flag should trump the user's global admin status in search requests

When doing searches we can specify an `includeExternal=true|false` flag which indicates whether or not resources from other tenants should be included in the response.

When a global admin performs a search we do not respect this parameter and simply query everything. We should apply the filter for global admins as well.
This will allow us to search through all the users of a tenant in case we need to make somebody a tenant admin.

A quick grep shows this needs to be changed in:
 * general search
 * related content

Assigning to @Coenego