extensionPrefs: Trick xgettext into accepting odd number of backticks

extensionPrefs: Trick xgettext into accepting odd number of backticks

Xgettext learned about template strings now, which is good. However
it's still buggy, so instead of the "classic" xgettext issue with
backticks, we now have exciting new issues to find work-arounds for.

One issue is that it doesn't detect backticks inside string constants
as regular characters, so having an odd number of backticks throws off
its regex.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/792