Refactor finders. Prevent circular dependency error

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/finders/base_finder.rb → app/finders/issuable_finder.rb

-# BaseFinder
+# IssuableFinder
 #
 # Used to filter Issues and MergeRequests collections by set of params
 #
@@ -16,7 +16,9 @@
 #     label_name: string
 #     sort: string
 #
-class BaseFinder
+require_relative 'projects_finder'
+
+class IssuableFinder
   attr_accessor :current_user, :params
 
   def execute(current_user, params)

app/finders/issues_finder.rb

@@ -15,7 +15,7 @@
 #     label_name: string
 #     sort: string
 #
-class IssuesFinder < BaseFinder
+class IssuesFinder < IssuableFinder
   def klass
     Issue
   end

app/finders/merge_requests_finder.rb

@@ -15,7 +15,7 @@
 #     label_name: string
 #     sort: string
 #
-class MergeRequestsFinder < BaseFinder
+class MergeRequestsFinder < IssuableFinder
   def klass
     MergeRequest
   end