Sentry sourcemaps do not work with element-desktop

[novocaine]

Sentry errors emitted by Element Desktop aren't being source-mapped, which means they aren't being grouped with the same exceptions on Element Web, and are harder to debug.

This seems to be because

1. The file paths in exception stack frames are prefixed with vector://vector/webapp/ where the hostname is on web
2. The bundle hash is also in the pathname, and it doesn't match web on the same release (not sure why this is)

I think we should strip the path prefixes including the bundle hash using https://docs.sentry.io/platforms/javascript/configuration/integrations/plugin/#rewriteframes - but I'm interested in why 2 is happening at all, it may lead to issues later on