add "Available" column to welcome page (#648)

add get_available_space() to NativeStorageServer

It uses a new 'available-space' key in the server's v1 version dict, or falls
back to 'maximum-immutable-share-size' (which presently always has the same
value but could have a different meaning in the future).

This is a squash merge of 9773555bb87fab71145ad7a0e84785a4e92d11f7
tests for "Available" column (#648)

thanks to the anonymous author who posted this on trac
add available-space to VERSION_DEFAULTS

as per warner's suggestion in https://github.com/warner/tahoe-lafs/pull/4
test_storage_client.py: add test_get_available_space_{old,new}.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>