Move custom software to our main Git repository
_Originally created by @intrigeri on [#7036 (Redmine)](https://public-redmine-archive.tails.boum.org/code/issues/7036)_
Blueprint: https://tails.boum.org/blueprint/Git_sub-repositories/
### Subtasks
- [x] Greeter: tails/tails#16912
- [x] pythonlib: tails/tails#16935
- [x] persistence-setup: tails/tails#17526
- [x] perl5lib: done via tails/tails#15281
- [x] iuk (Upgrader): done via tails/tails#15281
- [x] WhisperBack: tails/tails#16936
- [x] Installer: #17917
### Related issues
- **Related to** tails/sysadmin#5506
- **Related to** tails/tails#7087
- **Related to** tails/tails#9171
- **Related to** tails/tails#7221
- **Related to** tails/tails#15864
- **Related to** tails/tails#15778
- **Related to** tails/tails#16095
- **Related to** tails/tails#6442
- **Related to** tails/tails#10085
- **Related to** tails/tails#14516
- **Has duplicate** tails/tails#16678
- **Blocks** tails/sysadmin#12111
- **Blocks** tails/sysadmin#6218
- **Blocks** tails/sysadmin#6220
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