feat(sddm): add mount unit for installing sddm themes in Discover #283

Job	Run time
Build Stable Images (aurora) / image (main, aurora, stable)	17m 3s
Build Stable Images (aurora) / image (main, aurora-dx, stable)	26m 24s
Build Stable Images (bluefin) / image (main, bluefin, stable)	15m 2s
Build Stable Images (aurora) / image (nvidia, aurora, stable)	21m 34s
Build Stable Images (bluefin) / image (main, bluefin-dx, stable)	22m 26s
Build Stable Images (aurora) / image (nvidia, aurora-dx, stable)	29m 57s
Build Stable Images (bluefin) / image (nvidia, bluefin, stable)	19m 30s
Build Stable Images (bluefin) / image (nvidia, bluefin-dx, stable)	27m 1s
Build Stable Images (bluefin) / Check all builds successful	1s
Build Stable Images (aurora) / Check all builds successful	1s
Build Stable ISOs	0s
Generate Release / generate-release (stable)	1m 58s
Generate Release / generate-release (stable-daily)	40s
	3h 1m 37s

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