GUACAMOLE-1798: Add extension that supports connection templates.

[necouchman]

I've taken (another) run at this, this time using a decorating extension. Before I spend a bunch of time fixing up the myriad issues with this approach, I figured I'd solicit feedback on whether this is even a desirable approach, or if I should pursue some other route...or if it's something worth doing at all.

At present, it should:

• Present a selection box that lists the other connections the current user has access to, allowing you to select one of them as a "template".
• Merge the connection parameters from the selected "template" connection, overriding those with any set locally in that configuration, before connecting.
• Prevents you from selecting the connection you're editing as the template.

Among the several issues that need to be resolved, assuming this is a sane/desirable approach:

• Connections are not filtered by protocol, so you could "template" a SSH connection with a RDP connection, for example. Probably not ideal.
• Connection Attributes are not merged, only the GuacamoleConfiguration (parameters).
• Style/UX: Selection box isn't a constant size
• Style/UX: Selection box shows the paging at the bottom, rather than being a scrolling list.
• Style/UX: No indication on the page you're editing of parameter values from the template (not sure this is solvable with this particular method).

All that said, I'm putting it in draft mode, for now - I appreciate any thoughts/comments/guidance.