feat(badge): Implement UP/DOWN status badge (#291)

* Implement status badge endpoint

* Update integration tests for status badge generation

* Add status badge in the UI

* Update static assets

* Update README with status badge description

* Rename constants to pascal-case

* Check for success of the endpoint conditions

* Rename status badge to health badge

README.md

@@ -78,6 +78,7 @@ Have any feedback or want to share your good/bad experience with Gatus? Feel fre
   - [Exposing Gatus on a custom port](#exposing-gatus-on-a-custom-port)
   - [Badges](#badges)
     - [Uptime](#uptime)
+    - [Health](#health)
     - [Response time](#response-time)
   - [API](#api)
   - [High level design overview](#high-level-design-overview)
@@ -1407,6 +1408,23 @@ Example:
 If you'd like to see a visual example of each badge available, you can simply navigate to the endpoint's detail page.
 
 
+#### Health
+![Health](https://status.twin.sh/api/v1/endpoints/core_blog-external/health/badge.svg)
+
+The path to generate a badge is the following:
+```
+/api/v1/endpoints/{key}/health/badge.svg
+```
+Where:
+- `{key}` has the pattern `<GROUP_NAME>_<ENDPOINT_NAME>` in which both variables have ` `, `/`, `_`, `,` and `.` replaced by `-`.
+
+For instance, if you want the current status of the endpoint `frontend` in the group `core`, 
+the URL would look like this:
+```
+https://example.com/api/v1/endpoints/core_frontend/health/badge.svg
+```
+
+
 #### Response time
 ![Response time 1h](https://status.twin.sh/api/v1/endpoints/core_blog-external/response-times/1h/badge.svg)
 ![Response time 24h](https://status.twin.sh/api/v1/endpoints/core_blog-external/response-times/24h/badge.svg)