back button misbehaving in mirador

[eroux]

steps to reproduce:

• go to https://library.bdrc.io/show/bdr:W1KG10253
• click on the image to open the viewer (this opens the viewer but doesn't change the URL)
• the back button gets one step too far and goes the page before you went to the original page

[eroux]

also wrong repo, moved to https://github.com/buda-base/public-digital-library/issues/484