[databases] Serializer ships unindented declaration

This is a minor issue, but annoying. I'm trying to download a database from http://beatweb-staging and re-upload it to https://beat-eu.org/platform. While downloading it, I realise the database declaration (JSON) is not indented (with 4 spaces), which is the standard for the other contributions. The whole JSON comes in a single line of code.

I guess the issue here is the serializer that, contrary to what is in common/serializers.py, does not do any appropriate JSON dumping of the database declaration.

How to reproduce this issue:

$ ./bin/beat -p prefix -m http://beatweb-staging db pull livdet2013/1
...
$ cat prefix/databases/livdet2013/1.json
... #one single line of JSON file
$ ./bin/beat -p prefix -m http://beatweb-staging tc pull tutorial/eigenface/1
...
$ cat prefix/toolchains/tutorial/eigenface/1.json
... #perfectly indented