Detecting and setting a sensible UI scale

**Describe the bug**
Booting up elementaryos-6.0-daily.20210615.iso from a USB drive on a display with high density pixels the installer defaults to a 1.0 scaling factor making it really hard to read things.

**To Reproduce**
Steps to reproduce the behavior:
1. Put elementaryos-6.0-daily.20210615.iso on some data media
2. Boot up a machine to that disk
3. Wait for the installer to start

**Expected behavior**
UI of the installer to be of a physically-sensible size (buttons to be ~10mm tall, text to be >~5mm)

**Actual behavior**
UI is tiny. Scaling factor appears to be 1.0

**Screenshots**

**Desktop:**
- OS: Elementary (from live disk on USB drive)
- Browser: N/A
- Version: 6.0 beta 20210615

**Additional context**
Display is a Dell P2715Q (27", 3840x2160px, pixel pitch 0.1554mm) connected over DisplayPort