rm NativeStorageServer's unused min_shares argument
Merge pull request #126 from leif/remove-cruft

remove NativeStorageServer's unused min_shares argument
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>
wui: improved columns in welcome page server list

As discussed at https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1973

 - replace lengthy timestamps with human-readable deltas (eg 1h2m3s)
 - replace "Since" column with "Status" (addresses usability problem in #1961)
 - replace "announced" column with "Last RX" column
 - remove service column (it always said the same thing, "storage")

This is a squashed commit of 3fe9053134b2429904f673df561e602a50f83c7e
Thanks to an anonymous contributor who wrote some of the tests.