add server version to /api response

Fixes #171

design/api.md

@@ -86,6 +86,7 @@ The `application/json` response will have a JSON object as follows:
 
 *   `timeZoneName`: the name of the IANA time zone the server is using
     to divide recordings into days as described further below.
+*   `serverVersion`: the version of the server in use, eg `0.7.0`.
 *   `cameras`: a list of cameras. Each is a JSON object as follows:
     *   `uuid`: in text format
     *   `id`: an integer. The client doesn't ever need to send the id

server/src/json.rs

@@ -15,6 +15,8 @@ use uuid::Uuid;
 pub struct TopLevel<'a> {
     pub time_zone_name: &'a str,
 
+    pub server_version: &'static str,
+
     // Use a custom serializer which presents the map's values as a sequence and includes the
     // "days" and "camera_configs" attributes or not, according to the respective bools.
     #[serde(serialize_with = "TopLevel::serialize_cameras")]

server/src/web.rs

@@ -679,6 +679,7 @@ impl Service {
             req,
             &json::TopLevel {
                 time_zone_name: &self.time_zone_name,
+                server_version: env!("CARGO_PKG_VERSION"),
                 cameras: (&db, days, camera_configs),
                 user: caller.user,
                 signals: (&db, days),