add RSS functionality

[derbuur]

here an additional feature request with low priority:
add an rss function to the mission list so that user get informed if there is a new mission

[MorpheusXAUT]

Could be done via rss-package, but would most likely require some sort of caching combined with re-generation every X minutes.
Will take it into consideration, thanks for the suggestion!

[MorpheusXAUT]

Unfortunately, this cannot be added for user-specific missions unless some sort of "API token" authentication has been added. Simply providing a list of publicly visible missions (like an unauthenticated user would see) would be easier to provide, but obviously not complete.
The JWTs generated for user signup are not suitable for usage in e.g. a RSS reader, so a token provided via the URL would be of need.
If functionality is added to access all missions visible to a certain user, there must also be a place to revoke those tokens in case they have been shared by accident.