Addresses #263

Update the model to support snapshot visibility toggling
Addresses #263

Update lowlevel snapshotting code to save them in the right location
Addresses #263

Update low level nfs export manipulation code for snapshot visibility.
Fixes #263

Update nfs and snapshot views to support snapshot visibility
Merge branch 'issue#263' of https://github.com/schakrava/rockstor-core into schakrava-issue#263
Snapshots

Add visible flag to input form
Show errors using validator if json error msg is returned while creating
a snapshot
Remove deprecated share_detail.js file
Store export directory for the nfs export instead of share mount point in
export mount attribute.
Update replication code as required by other changes of #263
Fixes #261

When smart manager starts, do the necessary mounting and nfs exporting.
Return successful response for bootstrap when there are no exports.
Display errors correctly on snapshots and nfs exports tables
Merge branch 'schakrava-issue#263'