feat(api): enrich status with 'unavailable' stats

Closes reanahub/reana-ui#465

Author: CameronMcClymont

reana_commons/openapi_specifications/reana_server.json

@@ -1463,6 +1463,7 @@
               "application/json": {
                 "job": {
                   "available": 3,
+                  "unavailable": 1,
                   "health": "warning",
                   "pending": 3,
                   "percentage": 38,
@@ -1480,10 +1481,12 @@
                 },
                 "session": {
                   "active": 3,
+                  "unavailable": 1,
                   "sort": 3
                 },
                 "workflow": {
                   "available": 24,
+                  "unavailable": 6,
                   "health": "healthy",
                   "pending": 2,
                   "percentage": 80,
@@ -1501,6 +1504,9 @@
                     "available": {
                       "type": "number"
                     },
+                    "unavailable": {
+                      "type": "number"
+                    },
                     "health": {
                       "type": "string"
                     },
@@ -1550,6 +1556,9 @@
                     "active": {
                       "type": "number"
                     },
+                    "unavailable": {
+                      "type": "number"
+                    },
                     "sort": {
                       "type": "number"
                     }
@@ -1561,6 +1570,9 @@
                     "available": {
                       "type": "number"
                     },
+                    "unavailable": {
+                      "type": "number"
+                    },
                     "health": {
                       "type": "string"
                     },