## Summary

While testing elastic/integrations#5123 and
elastic#153216, I discovered two problems
with the list of categories on the Browse integration page:

1. The categories list doesn't respect `showIntegrationsSubcategories`
feature flag. When it is turned off, it will still only display
top-level parent categories. This PR fixes that so that when
`showIntegrationsSubcategories` is turned off, _all_ categories will be
listed on the left sidebar. When it is turned on, only the top-level
categories will be listed.
2. The merging of the categories list from EPR (i.e.
`<EPR_HOST>/categories`) with the categories hard-coded in Kibana for
custom integration cards is flawed: there can be a situation where a
category or subcategory is not returned by EPR, due to no EPR-hosted
integrations having it, but it is registered by a custom card. In this
case, the category is missing from the list. This PR fixes the merging
logic.

(cherry picked from commit 340ee10)