Update the migration guide and the FormHelper templates tags list
This will match modifications made in cakephp/cakephp#5435
Update global-constants-and-functions.rst

Removed a removed constant.
update StringTemplateTrait namespace

refs cakephp/cakephp#5407
Merge pull request #2142 from antograssiot/3.0

update StringTemplateTrait namespace
Merge pull request #2140 from davidyell/patch-14

Update global-constants-and-functions.rst
Merge pull request #2137 from HavokInspiration/formhelper-templates-tags

Update the migration guide and the FormHelper templates tags list