Replace mysql index limit logic with helper method for ee migrations

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

ee/db/migrate/20180308234102_add_partial_index_to_project_repository_states_checksum_columns.rb

@@ -11,7 +11,7 @@ class AddPartialIndexToProjectRepositoryStatesChecksumColumns < ActiveRecord::Mi
       add_concurrent_index(:project_repository_states,
         [:repository_verification_checksum, :wiki_verification_checksum],
         name: INDEX_NAME,
-        length: Gitlab::Database.mysql? ? 20 : nil,
+        length: mysql_compatible_index_length,
         where: 'repository_verification_checksum IS NULL OR wiki_verification_checksum IS NULL'
                           )
     end

ee/db/migrate/20180314174825_add_partial_index_to_project_repository_states_verification_columns.rb

@@ -9,11 +9,11 @@ class AddPartialIndexToProjectRepositoryStatesVerificationColumns < ActiveRecord
 
   def up
     unless index_exists?(:project_repository_states, :last_repository_verification_failure, name: REPOSITORY_INDEX_NAME)
-      add_concurrent_index(:project_repository_states, :last_repository_verification_failure, name: REPOSITORY_INDEX_NAME, length: Gitlab::Database.mysql? ? 20 : nil, where: 'last_repository_verification_failure IS NOT NULL')
+      add_concurrent_index(:project_repository_states, :last_repository_verification_failure, name: REPOSITORY_INDEX_NAME, length: mysql_compatible_index_length, where: 'last_repository_verification_failure IS NOT NULL')
     end
 
     unless index_exists?(:project_repository_states, :last_wiki_verification_failure, name: WIKI_INDEX_NAME)
-      add_concurrent_index(:project_repository_states, :last_wiki_verification_failure, name: WIKI_INDEX_NAME, length: Gitlab::Database.mysql? ? 20 : nil, where: 'last_wiki_verification_failure IS NOT NULL')
+      add_concurrent_index(:project_repository_states, :last_wiki_verification_failure, name: WIKI_INDEX_NAME, length: mysql_compatible_index_length, where: 'last_wiki_verification_failure IS NOT NULL')
     end
   end
 

ee/db/post_migrate/20180605213516_fix_partial_index_to_project_repository_states_checksum_columns.rb

@@ -23,7 +23,7 @@ class FixPartialIndexToProjectRepositoryStatesChecksumColumns < ActiveRecord::Mi
     add_concurrent_index(:project_repository_states,
       [:repository_verification_checksum, :wiki_verification_checksum],
       name: OLD_INDEX_NAME,
-      length: Gitlab::Database.mysql? ? 20 : nil,
+      length: mysql_compatible_index_length,
       where: 'repository_verification_checksum IS NULL OR wiki_verification_checksum IS NULL'
                         )
   end