Improve styling of keywords with extra functionality such as "Closes"

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Problem

Currently Closes #1 when written down in a MR is not very clear that it has special meaning. Except when you see it appear in the MR widget. GH introduced a simple styling method to improve that https://github.com/blog/2398-better-manage-issues-and-pull-requests-with-keyword-updates

Proposal

I think we should do the same here.. it seems like a simple win.

Design

has styling:

    text-decoration-line: underline;
    text-decoration-style: dotted;

Renders tooltip with text: This merge request will close the following issue(s)

cc: @sarrahvesselov @victorwu