[courier/rootSearchSource] stop loading the defaultIndexPattern

[spalger]

Kibana uses a rootSearchSource to define the global search context (things like the time filter). This searchSource has historically been assigned the default index pattern, but as far as I can tell this behavior is not utilized or desired and this PR removes it. Now searchSources must define the index they are intended for in order to be fetch-able.

[w33ble]

This appears to close #9028

EDIT: Maybe not actually... I can't get 9028 to happen locally at all, even in master, so I don't think this actually closes it. But it may already be closed.

[w33ble]

@spalger I'm not sure what this is fixing exactly. I can't get an error state in master that this would seem to resolve. How should we be testing this fix? And does it close any open issues?

[spalger]

@w33ble @BigFunger This doesn't outright fix any issue, it just removes an "optimization" in favor of only loading IndexPatterns that we actually need. This was mentioned by a user who was pointing out that even when Kibana didn't use the "default" index pattern it tried to load it, which is slightly wasteful but in a corner case, when mixed with document-level access control and #9028 (and related issues), lock users out of Kibana unnecessarily.

Consider this scenario:

1. Users 1 and 2 have access to Index A
2. Default Kibana IndexPattern is Index A
3. User 1 builds a bunch of dashboards to User 2 based on Index A
4. User 1 introduces User 3, creates Index B, gives access to themselves and User 3
5. While User 1 builds dashboards for User 3 they change the default IndexPattern to Index B
6. User 2 can't use Kibana anymore because they don't have access to the IndexPattern document for Index B.

This doesn't fix the issue if the user tried to use the index pattern, or loaded up discover with a default index, but it does remove the useless loading of the default index pattern.

[BigFunger]

@spalger can you rebase/merge on master, please?

[BigFunger]

LGTM

[w33ble]

Thanks for the explanation. LGTM.