UI: add abstract elements to the topology map drawing

[italovalcy]

Hi,

Sometimes, the link on Kytos is not a direct/physical link dedicated to Kytos. It may also be an ethernet pseudo-wired, it can be a link transported through multiple optical equipment, etc. For some network operators, it would be nice if we could draw "abstract elements" into the Kytos topology map to represent such scenarios.

By abstract elements I mean basically an item which its only purpose is to be displayed on the topology. We will draw this element at specific coordinates (lat and lng, provided by the network operator), and then links can be created from and to this element. Since we are using blue circles for actual switches, we could use other representation for abastrct elements (e.g, gray squares, etc)

After adding those abstract elements, we should be able to configure their geo-coordinantes and who they will link to.

The current way we display links will also change: instead of a link just connecting Link.endpoint_a to Link.endpoint_b, we should first check for the existence of abstract elements in between them and connect the dots.

Example:

Observe that in this drawing above, we have two abstract elements: R1-Router and Router MX1. The first one made the lines for the link between PTY-TON-SWO1 and JAX-CLK-SW04 to be different from its original place.