Move feedback formatting to alert service + make documentediting use it

[asaunier]

Please note that errors triggered when trying to save a document without being authenticated cannot be managed the right way because the API does not return a correct error message, see c2corg/v6_api#85

[asaunier]

The main changes of this PR are:

• formatErrorMsg_ and filterStr_ functions are moved to the alerts service since useful for most alerts (not only auth alerts).
• the documentediting tool + the logout button also use the alerts service.

[asaunier]

Any objection?

[asaunier]

Could anyone review this PR please?
I need it for PR #116

[tsauerwein]

@gberaudo would want a type for this. ;)

[asaunier]

Yes, I also expected @gberaudo would say something here :p
That would make sense indeed, especially since the alerts service expects this is a response object with a response['data']['errors'] property. But are the API responses always formatted the same way? I'm also afraid that the type is a bit complicated.

[tsauerwein]

Two small comments, otherwise it looks ok.

[asaunier]

Perhaps I should add an assert here to make sure that response['data']['errors'] is available.

[asaunier]

Two small comments, otherwise it looks ok.

Thanks for reviewing @tsauerwein !
I will make the few changes I have just mentioned and will merge if there is no other objections.