Don't fetch mappings at index pattern creation

[w33ble]

Kibana version: Any

Browser OS version: Any

As noted in #5818, the flow around index pattern creation is kind of weird, and simply loading that page can potentially take down a cluster. This is due to the query for _mappings of the default pattern on load.

We shouldn't make that query by default, it really should require some interaction from the user. I mentioned this in #6453, but wanted to create an issue for it.

Steps to reproduce:

1. Open Kibana for the first time
2. Note the request to logstash-*/_mappings when you are redirected to the index pattern creation screen

[chrisronline]

I'm fairly sure this was fixed with recent changes to the index pattern creation page