Add default_branch_name column to namespace_settings table

What does this MR do?

Adds default_branch_name to namespace_settings to support allowing group admins to specify a default branch name for their projects.

db/migrate/20200919200318_add_default_branch_name_to_namespace_settings.rb

up

== 20200919200318 AddDefaultBranchNameToNamespaceSettings: migrating ==========
-- add_column(:namespace_settings, :default_branch_name, :text)
   -> 0.0012s
== 20200919200318 AddDefaultBranchNameToNamespaceSettings: migrated (0.0012s) =

down

== 20200919200318 AddDefaultBranchNameToNamespaceSettings: reverting ==========
-- remove_column(:namespace_settings, :default_branch_name, :text)
   -> 0.0032s
== 20200919200318 AddDefaultBranchNameToNamespaceSettings: reverted (0.0082s) =

db/migrate/20200919204155_add_text_limit_to_namespace_settings_default_branch_name.rb

up

== 20200919204155 AddTextLimitToNamespaceSettingsDefaultBranchName: migrating =
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE namespace_settings\nADD CONSTRAINT check_0ba93c78c7\nCHECK ( char_length(default_branch_name) <= 255 )\nNOT VALID;\n")
   -> 0.0012s
-- execute("SET statement_timeout TO 0")
   -> 0.0001s
-- execute("ALTER TABLE namespace_settings VALIDATE CONSTRAINT check_0ba93c78c7;")
   -> 0.0012s
-- execute("RESET ALL")
   -> 0.0002s
== 20200919204155 AddTextLimitToNamespaceSettingsDefaultBranchName: migrated (0.0122s)

down

== 20200919204155 AddTextLimitToNamespaceSettingsDefaultBranchName: reverting =
-- execute("ALTER TABLE namespace_settings\nDROP CONSTRAINT IF EXISTS check_0ba93c78c7\n")
   -> 0.0013s
== 20200919204155 AddTextLimitToNamespaceSettingsDefaultBranchName: reverted (0.0072s)

Closes #251935 (closed)