Notify user about existence of pasted URL

[kirillt]

ARK-Builders/ARK-Shelf#24
User should get input box highlighted when they paste URL which is already among saved links. Attempt to save such a link should result in an error message, or edit dialog spawned with older version of the link.

[kirillt]

Open questions:

• just match resource ids or compare URLs?
• should we strip query parameters before comparison?

[gwendalF]

@kirillt Is there a difference between ressource ids or URLs? It depends the behavior but I think we should keep full URL because it can resolve in different pages. For examples if query is used to filters some result may be that"s the result we are interested in

[kirillt]

@gwendalF basically resource ids approximate URLs pretty well, i.e.:

1. if id1 = id2 then url1 = url2 with 99.99% chance
2. if id1 ≠ id2 then url1 ≠ url2 with 99.99% chance

We can ignore potential collisions for this moment. Later, we'll create more sophisticated ResourceId in arklib.

I also think we should not strip query parameters. I had this idea, but URL can change significantly if we remove parameters from it. Better to implement grouping by domain.