Add selected topics to URL on graph page

[bracyw]

Description

It would be nice to share links with certain topics pre-selected for the graph screen. Currently everything is done via selection through a tracked variable (soon to be through a service, but still just wrapping a variable), and is lost on reload. We should make it so that when a topic is select it is added to the url on the graph page.

Currently blocked by #451

Acceptance Criteria

• selection works like before.
• when a topic is selected it is added to url
• when a topic is deselected it is removed from url
• if you paste a url of selected topics, they are selected on the graph screen, and the tree of selected topics opens up accordingly.

Proposed Solution

Use the angular Router, and TBD graph topic service.

Mocks

No response