Issue 131

Provide two options for each Account:
Compact (which VACUUMs the accounts and prunes recreatable
attachments)

Clear all data (danger!) (which wipes all messages and attachments in
the account, except placeholder deleted messages)

Displays a Toast when the Compact or Clear is queued, and another
Toast with the shrinkage results.

(Also, a small change to increase probability of seeing messages after
a sync, by setting the needsRefresh on all folders when a sync is done.)