Remove _all settings in the 5.x version of the template

[tsg]

In the 5.x version of the template we currently have:

"_all": { "norms": false}

This breaks after upgrading ES to 6.0, without upgrading the template.

~~We should replace it with:

"_all": { "enabled": false }

Disabling _all is fine, because ES 5.X supports searching all fields without the need of an _all field.~~

We should do this in both 6.0 and 5.6 branches. This will help with the upgrade process.

• 5.6 branch Remove the _all settings from the 5.x template (5.6 branch) #4902
• master + 6.0 branch Remove the _all field settings from the 5.x template (master/6.0 version) #4928
• Backport Remove the _all field settings from the 5.x template (master/6.0 version) #4928 to 6.0 Cherry-pick #4928 to 6.0: Remove the _all field settings from the 5.x template (master/6.0 version) #4948
• Include notice about the breaking change regarding the _all field in 6.0 Added breaking change CHANGELOG entry for the removal of the _all field #4947

[tsg]

There's an issue with disabling _all:

With _all enabled, searching by one word of a keyword field was working, because all fields were indexed as text in _all. With _all disabled, one would have to search by the full field value, individual words don't match any document.

This leads to two issues that we should discuss:

• If we disable the _all fields in 5.6, it's a breaking change

• The _all option doesn't exist in 6.0, we'd need to explicitly simulate it using copy_to to a single text field.

At the very minimum, we'll have to include a line in Breaking changes about this change in behavior.

[tsg]

The new plan, due to the issue above, is to remove the _all settings from the 5.6 version of the template. This costs a byte per document, but it's the safest approach.

I'm updating the PRs to reflect this decision.