[CRAWL] In a crawl's startpages, one cannot add a startpage after having removed another

[boogheta]

How to reproduce:

• prepare a crawl (either from Import urls, Prospect or List WebEntities)
• click on the crawl box to open the startpages modal
• click on a cross to remove one if the existing startpages
• enter an url and validate it to add a new startpage => bug 1) it does not add it to the displayed list (although it is stored and the total number increases on the bow in the background)
• enter the same url again and validate to add it => bug 2) it opens a "check new startpages" stuck on 0% (probably due to the fact the url is not in the displayed list whereas it should, so fixing bug 1) shall probably fix bug 2) at the same time)