Use relative paths and support installing in subfolders

[agowa]

Is your feature request related to a problem? Please describe.

deploying for a subpath like "/chat" currently doesn't work. Even if the subpath option within the webpack configuration is used.

Describe the solution you'd like

Only use relative paths or allow to configure the subpath before building.

Describe alternatives you've considered

Using another matrix web client.

Additional context

I'm currently rebuilding my webpage and I want to host it within IPFS and I'd like to put the matrix chat app within a subpath of "/chat".
For the IPFS requirement, it would be nice if the path is relative because otherwise, the page won't work if the IPFS browser plugin is installed and clients get redirected to the gateway URL.
I'm also considering hosting my webpage on multiple domains (normal, ipfs, i2p, tor)

[ajbura]

Related: #27