[Sync] Flush permalinks on successful sync

[brettsmason]

There have been a few issues recently with failed uploads or editor errors which have been down to permalinks needing to be flushed. I suggest we add wp rewrite flush to the sync scripts to hopefully alleviate this.

[edjeavons]

I've added a post-sync operation that runs all of the following, just to be extra thorough:

wp rewrite flush
wp transient delete --all
wp cache flush