Enhancing Metabase Manager with addition of authorisation, aggregation and server addition features

[ashuchow]

I have added a few features in this pull request that should help with metabase manager being a lot more helpful:

• Set up a local authorisation feature that has a root superuser that when logged in can allow for setting credentials for other users, with a toggle for whether they are allowed the same privileges to create other users or not. Feature available from the Add Users page in the Navigation bar.
• Each user can now add and delete servers for metabase instances that they would like to manoeuvre through and sync across within their own accounts, with the option to toggle them as host servers or destination servers. Feature/page available from the Add Servers button in the Navigation bar.
• A separate page to execute SQL queries from all the servers that have been added by the user has been set up. All the toggled servers return their respective results for the executed query and the results can be downloaded in a consolidated Excel sheet. Feature/page available from the Query Creator button in the Navigation bar.
• The main page now has a drop down menu for both host server and destination server components so that servers added for a given user are accessible to autofill into the form along with the required credentials.
• Embedded views of the public metabase dashboards that are currently being used by CARE. Pages available from the Public Dashboards button in the Navigation bar.