UI: add JumpGate#endpoint_title helper

Server returns jump gates w/ endpoint_id not endpoint systems
themselves, which clients need to lookup details for if desired.

Adds & uses helper to js JumpGate to always return displayable
text given a jump gate, preferibly its endpoint human-readable
name, else the endpoint_id if that hasn't been retrieved yet
(or the server hasn't responded yet)

Tests need to be added