In version 2.0.15 `gotify` switched to `packr` at 2.x which is why the
UI can't be served properly via HTTP and causes an empty 500 response and
the following errors in `journald`:
```
2020/09/12 19:18:33 [Recovery] 2020/09/12 - 19:18:33 panic recovered:
GET / HTTP/1.1
Host: localhost:8080
Accept: */*
User-Agent: curl/7.72.0
stat /home/ma27/Projects/ui/build/index.html: no such file or directory
```
This wasn't caught by the VM-test as it only tested the REST and push
APIs. Using their internal `packr.go` script in our build as it's the
case in the upstream build-system[1] fixes the issue.
[1] https://github.com/gotify/server/pull/277/files#diff-b67911656ef5d18c4ae36cb6741b7965R48
