feat: ability to serve static files (#1174)

Added the configuration key, `service.staticpath`, to serve files from the configuration path on root (/).

Serving static files allows the api service to also serve the frontend content. This is a simple option for deploying Vikunja without needing any other servers or proxies.

Running a complete instance becomes:

    VIKUNJA_SERVICE_STATICPATH=/path/to/frontend ./vikunja

Where `/path/to/frontend` is a copy of Vikunja's frontend static files.

## Implementation

Providing a path, via the configuration or environment, adds a static file middleware to serve the path's contents from root (/).

By default, the configuration path is empty and Vikunja's existing behaviour is unchanged.

Co-authored-by: Graham Miln <graham.miln@dssw.co.uk>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1174
Reviewed-by: konrad <k@knt.li>
Co-authored-by: grahammiln <grahammiln@noreply.kolaente.de>
Co-committed-by: grahammiln <grahammiln@noreply.kolaente.de>

config.yml.sample

@@ -22,6 +22,8 @@ service:
   # Vikunja will also look in this path for a config file, so you could provide only this variable to point to a folder
   # with a config file which will then be used.
   rootpath: <rootpath>
+  # Path on the file system to serve static files from. Set to the path of the frontend files to host frontend alongside the api.
+  staticpath: ""
   # The max number of items which can be returned per page
   maxitemsperpage: 50
   # Enable the caldav endpoint, see the docs for more details