Add templated table for mobile screenshots to default MR template

What does this MR do and why?

To encourage developers to think about smaller viewport experiences and provide a convenient means of formatting screenshots for them, this MR adds a (commented-out) optional table with a column for our different breakpoints and a before and after column.

If they wish, an MR author can un-comment the table and populate it with screenshots that show how their changes behave at different breakpoints.

The table looks like this:

Viewport size	Before	After
xs (<576px)
sm (>=576px)
md (>=768px)
lg (>=992px)
xl (>=1200px)

MR acceptance checklist

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