Refactor MarkdownDialog.vue to ts+composition. #2525

Job	Run time
build-client / build-client	10m 12s
setup-selenium / setup_chromedriver	3s
Test (3.8, 0)	44m 55s
Test (3.8, 1)	55m 25s
Test (3.8, 2)	49m 40s
	2h 40m 15s

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