Frontend calls the GET /apps/tickets endpoint too often

[drik98]

Currently the frontend calls the GET /apps/tickets endpoint every few seconds. Probably when the /apps event source gets updated. May we can improve that.

Possible options:

(1) Add eTags to at least improve the caching in the frontend
(2) Embed the ticket information directly to the /apps/ endpoint and completely remove the necessity to call the tickets endpoints
(3) see #275 (comment)
(4) Adjust the logic in the frontend to only request the /apps/ticket resource when the preview names have changed and not when just the status of service changes? Though I am not sure if that is actually the logic that is in place now.

[schrieveslaach]

(3) stream the ticketing information

Recently, I reduced already the calls to Jira itself (see #270, and the commit 93c5723 on mainline). In the context of that I was thinking, that PREvant also should stream changes to the ticketing information to the frontend. I would go for option (3) because one gets the recent ticketing information without refreshing the browser which is similar to streaming the list of applications.

Nevertheless, adding ETag should be also possible due to #270