Replace "Shutdown" with "Shut Down" in user-facing strings

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