Improvements to remote host management

Main changes:

1. Added host table to manage remote hosts, works on mongo and sqlite.
2. Added host addition capability to Settings page. Currently add and delete is supported. No update to host data like username, key or password.
3. Better error handling for remote tasks.

Things to improve:
1. Ability to change a host for a task from "Remote" tab in the Job details page.
2. Write reset table function for Settings page when a host is added or deleted.