API: Add POST method for listing apps

Created by: mardy

Add an api/v2/apps endpoint accepting a POST request containing a JSON-formed body carrying the list of application IDs to be used as additional filter. Since the list of supported frameworks can also grow quite big, move that to the body, too.

With this change, the API is nearly identical to the one used by the Ubuntu store.

This is a proposal to fix https://github.com/UbuntuOpenStore/openstore-meta/issues/156

Note that I've added support for only the apps and frameworks parameters in the body. I did so to keep the code simpler, and minimize the changes (the other parameters don't require lots of space and can continue to be passed in the query string), but it might be better to support them all in the body, for consistency. Please let me know, and if you prefer so I'll update this branch.