openapi: Migrate response descriptions into schemas.
openapi: Migrate response descriptions into schemas.

Currently, the descriptions of API responses were
hardcoded in the template. However, as a part of the
goal of moving towards a common template, these should
be auto-generated.

This commit copies these descriptions into the `description`
parameter in the OpenAPI data for that effect.

rht/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,hackerkid/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,andersk/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,andersk/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,punchagan/zulip,zulip/zulip,punchagan/zulip,andersk/zulip