Thanks uses an outdated PNG for an icon graphic. It also has issues with alignment.
Let's modernise this to an SVG to avoid this problem.
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= Acceptance criteria
[x] Use mw-ui-icon for generating the icon
[x] Update icon to [[ https://doc.wikimedia.org/oojs-ui/master/demos/dist/themes/wikimediaui/images/icons/userTalk-ltr.svg | new asset ]]
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[x] Remove CSS code in MobileFrontend that workarounds the lack of usage of mw-ui-icon
= QA steps
General QA of the thanks on diff feature - make sure you are happy with the icon and the behaviour.
[] Login and visit a diff e.g. https://en.m.wikipedia.beta.wmflabs.org/wiki/Special:MobileDiff/81611 or https://en.m.wikipedia.beta.wmflabs.org/wiki/Special:MobileDiff/381373
[] Thank the user