Added GitHub action to do axe accessibility tests with [axe
cli](https://github.com/dequelabs/axe-core-npm/tree/develop/packages/cli).
I believe it is best by default to let this be run only manually, since
fixing some of the issues are not straightforward (I haven't fixed them
all for this template yet).
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Signed-off-by: George Araújo <george.gcac@gmail.com>
George