Sort members list by last_sign_in_at

[sseerrggii]

In this commit b0d675c#diff-4e05ad0d64e6100656b63ad1e78f32c5R6 @mllocs try to sort members in /members by last_sign_in_at but seems that ransack have troubles to sort null results at the end.

For that we revert the change 9208d17

Can we fix it? Will be good to see different members in this main page /members

Also is better to have members sorted by activity (Sorted by date of last transfer of member)

[sseerrggii]

@markets can you help here?

Only this issue remains to be done on second quarter of 2019 💌

[markets]

@sseerrggii work started in 👉 #495