Create a dedicated accounts/users page in the web UI to view/manage users

[dstpierre]

Since 5351407 the web UI database page is not displaying the system collections, the one prefixed with sb_ owned by StaticBackend.

In #26 there were an item regarding adding a dedicated page for user management. This is its dedicated issue.

I'd see a simple table listing the sb_accounts and when clicking on an account we would see its users from sb_tokens either directly in the same page or in a new page for this account.

There's currently no way to list accounts and users from an account. Here's how I'd approach this:

1. In persistor.go I'd add two new functions in the // system user account function s section. ListAccounts and ListUsers
2. An implementation for the postgresql, mongo, and memory providers will be required.
3. In the ui.go create two new hanglers like listAccounts and listUsers. The listAccounts does not need much parameters, but listUser will require an accountId.
4. Add the two route in server.go line:227 near webUI := ui{} block of routes.

Additional thoughts.

• Could be nice to be able to search by email for an account / user in that page.
• Ideally, the account would be sorted by last created.