Draft: Add `default_merge_method` to ApplicationSettings model

Part of #35266

What does this MR do?

Add column default_merge_method to application_settings

UP

== 20210331114710 AddDefaultMergeMethodToApplicationSettings: migrating =======
-- add_column(:application_settings, :default_merge_method, :text, {:default=>"merge", :limit=>nil, :null=>false})
   -> 0.0072s
== 20210331114710 AddDefaultMergeMethodToApplicationSettings: migrated (0.0073s) 

== 20210331114912 AddLimitToDefaultMergeMethodToApplicationSettings: migrating 
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0004s
-- execute("ALTER TABLE application_settings\nADD CONSTRAINT check_74a73b6cd8\nCHECK ( char_length(default_merge_method) <= 16 )\nNOT VALID;\n")
   -> 0.0036s
-- current_schema()
   -> 0.0003s
-- execute("SET statement_timeout TO 0")
   -> 0.0008s
-- execute("ALTER TABLE application_settings VALIDATE CONSTRAINT check_74a73b6cd8;")
   -> 0.0088s
-- execute("RESET ALL")
   -> 0.0010s
== 20210331114912 AddLimitToDefaultMergeMethodToApplicationSettings: migrated (0.0379s) 

DOWN

== 20210331114912 AddLimitToDefaultMergeMethodToApplicationSettings: reverting 
-- execute("ALTER TABLE application_settings\nDROP CONSTRAINT IF EXISTS check_74a73b6cd8\n")
   -> 0.0017s
== 20210331114912 AddLimitToDefaultMergeMethodToApplicationSettings: reverted (0.0156s) 

== 20210331114710 AddDefaultMergeMethodToApplicationSettings: reverting =======
-- remove_column(:application_settings, :default_merge_method, :text, {:default=>"merge", :limit=>nil, :null=>false})
   -> 0.0036s
== 20210331114710 AddDefaultMergeMethodToApplicationSettings: reverted (0.0054s) 

Add column default_merge_method to namespace_settings

UP

== 20210331154810 AddDefaultMergeMethodToNamespaceSettings: migrating =========
-- add_column(:namespace_settings, :default_merge_method, :text, {:default=>"merge", :limit=>nil, :null=>false})
   -> 0.0045s
== 20210331154810 AddDefaultMergeMethodToNamespaceSettings: migrated (0.0046s) 

== 20210331154932 AddLimitToDefaultMergeMethodToNamespaceSettings: migrating ==
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE namespace_settings\nADD CONSTRAINT check_874179f86a\nCHECK ( char_length(default_merge_method) <= 16 )\nNOT VALID;\n")
   -> 0.0013s
-- current_schema()
   -> 0.0003s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- execute("ALTER TABLE namespace_settings VALIDATE CONSTRAINT check_874179f86a;")
   -> 0.0068s
-- execute("RESET ALL")
   -> 0.0006s
== 20210331154932 AddLimitToDefaultMergeMethodToNamespaceSettings: migrated (0.0323s) 

DOWN

== 20210331154932 AddLimitToDefaultMergeMethodToNamespaceSettings: reverting ==
-- execute("ALTER TABLE namespace_settings\nDROP CONSTRAINT IF EXISTS check_874179f86a\n")
   -> 0.0012s
== 20210331154932 AddLimitToDefaultMergeMethodToNamespaceSettings: reverted (0.0147s) 


== 20210331154810 AddDefaultMergeMethodToNamespaceSettings: reverting =========
-- remove_column(:namespace_settings, :default_merge_method, :text, {:default=>"merge", :limit=>nil, :null=>false})
   -> 0.0024s
== 20210331154810 AddDefaultMergeMethodToNamespaceSettings: reverted (0.0036s)