MM-61991 Show server hostname in about modal (#29413)

This introduces a new entry in the `Main Menu -> About` modal with the hostname of the currently connected websocket. This will be used to aid debugging issues in clustered environments by showing which node in the cluster is servicing requests for a particular websocket.

This information is only visible in self-managed instances. It will not be visible on cloud instances.

webapp/channels/src/plugins/pluggable/__snapshots__/pluggable.test.tsx.snap

@@ -124,6 +124,7 @@ exports[`plugins/Pluggable should match snapshot with extended component 1`] = `
           "reconnectListeners": Set {},
           "responseCallbacks": Object {},
           "responseSequence": 1,
+          "serverHostname": "",
           "serverSequence": 0,
         }
       }
@@ -262,6 +263,7 @@ exports[`plugins/Pluggable should match snapshot with extended component with pl
           "reconnectListeners": Set {},
           "responseCallbacks": Object {},
           "responseSequence": 1,
+          "serverHostname": "",
           "serverSequence": 0,
         }
       }
@@ -534,6 +536,7 @@ exports[`plugins/Pluggable should match snapshot with null pluggableId 1`] = `
           "reconnectListeners": Set {},
           "responseCallbacks": Object {},
           "responseSequence": 1,
+          "serverHostname": "",
           "serverSequence": 0,
         }
       }
@@ -673,6 +676,7 @@ exports[`plugins/Pluggable should match snapshot with valid pluggableId 1`] = `
           "reconnectListeners": Set {},
           "responseCallbacks": Object {},
           "responseSequence": 1,
+          "serverHostname": "",
           "serverSequence": 0,
         }
       }