Simplify redundant fallbacks

Fix #261

fix \ProvidesPackage{} for highcontrast theme

Format table rules following booktabs advice

Removes warning:

LaTeX Warning: You have requested package `beamercolorthememetropolis-highcontrast',
               but the package provides `beamercolorthememetropolis-minimal'.

Simplify a patch

This avoids looking for an internal command that will be removed
in a future update.

Update patch: track beamer changes

Drop braces in input line

The fixes in
https://github.com/josephwright/beamer/commit/ef419391cbe5245cabf3b77c89b58023fb253c95
and
https://github.com/josephwright/beamer/commit/ce790e640c0b41ab3adf764a79a9ef9475e37532
mean that a new patch route is needed. Patches cannot be nested
(easily) so use two and a switch to track if an error is needed.

The command needs three arguments: \@ehc is the 'Press Enter to
continue" generic advice.

Add metropolis-highcontrast color theme