[Feature] Install as progressive web app.

[Leopere]

Describe the solution you'd like
There is a way to do this such that it's just offered by the Omnibar in Google Chrome / Variants but something along the lines of this.
https://www.ghacks.net/2018/10/18/how-to-install-progressive-web-apps-in-google-chrome/
go-gitea/gitea#4730
https://blog.gitea.io/2019/01/gitea-1.7.0-is-released/

Describe alternatives you've considered
At some point, it sounds like @StashAppDev wanted to add an iOS app or Android app.