webui: reduce number of API requests

[milahu]

the webui is not just slow because javascript
but also because it has to send a separate api request for each torrent

ideally it should take only one api request
to fetch all the "important" data of all torrents
to render the initial view

and it would be nice to have
api requests for incremental updates
with an option to select only some torrents

all the "important" data

the API could be generalized with graphQL