Summary:
"Shutdown" is a noun ("The nuclear reactor is experiencing a shutdown!")

"Shut Down" is an imperative command with a verb ("Shut down the nuclear reactor!"), which is how everything else is phrased.

Test Plan:
- Login screen button now says "Shut Down"
- Searching for "shut down" in KRunner brings up the appropriate option ("shutdown" still works too)
- KRunner says, "Shut down the computer"

Reviewers: #vdg, #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15878