Add annotation to limit body size of incoming requests

[LukasReschke]

As suggested by @blizzz, we should consider adding an annotation to the AppFramework that limits the incoming body size. This can help in cases where we could accidentally store too much data in the database.

The annotation could by default be set to a reasonably high limit such as 1MB, and the limit should be overwritable by developers. (or even completely disabled)

[LukasReschke]

So this is slightly trickier than expected as there doesn't seem to be a straight forward way to access the size of the request without reading it fully. I'd however like to avoid reading the whole request into once before hitting the AppFramework. (my first intuition was writing an AppFramework middleware)

Other potential approaches:

• Throw the exception when reading the variable inside Request.php of the AppFramework
• ...?

[LukasReschke]

(instead of a full strlen we could also do an isset on the element which could speed up things)