This commit removes the ability to use string fields on indices created on or
after 5.0. Dynamic mappings now generate text fields by default for strings
but there are plans to also add a sub keyword field (in a future PR).
Most of the changes in this commit are just about replacing string with
keyword or text. Some tests have been removed because they existed because of
corner cases of string mappings like setting ignore-above on a text field or
enabling term vectors on a keyword field which are now impossible.
The plan is to remove strings entirely in 6.0.
eef19be072