The Greeter's release process is broken: POTFILES.in lists non-existing file

Originally created by @intrigeri on #12200 (Redmine)

… and there are no instructions to generate it:

./setup.py build_i18n                                                         
running build_i18n
intltool-update -p -g tails-greeter
xgettext: error while opening "../data/greeter.ui.h" for reading: No such file or directory
ERROR: xgettext failed to generate PO template file. Please consult
       error message above if there is any.
error: command 'intltool-update' failed with exit status 1

I assume that commit c3ee13f91e5ef25b0b10e4353538e4cdcc781afc was correct, but then wiki/src/contribute/release_process/tails-greeter.mdwn needs to be updated in the feature/stretch branch so that one can follow it without having to guess what additional steps are required.

Thanks in advance!

Parent Task: #8230 (closed)