api_docs: Have user settings with enum integers use schema keyword.
api_docs: Have user settings with enum integers use schema keyword.

There are three user settings that are integer enums:
`color_scheme`, `demote_inactive_streams` and
`desktop_icon_count_displays`.

Unlike the other user settings, these were using the `content`
keyword instead of the `schema` keyword in their definitions,
which caused them not to be rendered correctly in the api
documentation.

Changes the keyword to `schema` and fixes the indentation
for these three user settings in the two endpoints using
them.

zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,kou/zulip,zulip/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,kou/zulip,zulip/zulip,andersk/zulip,andersk/zulip,kou/zulip,rht/zulip,kou/zulip,kou/zulip,zulip/zulip,rht/zulip,andersk/zulip,kou/zulip,kou/zulip,zulip/zulip,andersk/zulip