Update dependency mini-css-extract-plugin to v0.12.0

This MR contains the following updates:

Package	Type	Update	Change
mini-css-extract-plugin	devDependencies	minor	0.5.0 -> 0.12.0

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mentioned in issue #1143 (closed)

merged

mentioned in commit 9be06d9f

cc @tusooa this is merged now, should be fine for !1425 (merged) right?