Third-party URL shortener support

[dlrudie]

I have my Zipline instance at https://zipline.regulardomain.com

However, I already host a shlink URL shortener at https://123.45

I would like to be able to integrate that URL shortener instead as it is significantly shorter.
i.e.
https://zipline.regulardomain.com/u/123456.jpg
vs
https://123.45/12345