Persist sequence opened in modal in URL

[chaoran-chen]

Related to #2024, I think that it would be very useful to persist the sequence entry ID in the URL when it is opened in a modal on the browse page. This would make it easier for people to send a link to an entry (now, you have to first open it in a new tab and not every user will know that). Further, it will re-open the same entry if the page is reloaded (which one might do on purpose but also, for example, upon restart of a browser).

[theosanderson]

We can definitely do this. It gets into a slightly confusing state where there is the potential for the modalised sequence to no longer be visible on the search page, because the search results have changed. If we have for example left and right arrow keys to swap between what is displayed in the modal, then it's not clear what they should do in that case (probably nothing) - but that's not a huge deal.