This adjusts the bundled plugins to do their admin permission checks
based on their admin component's isAccessibleByCurrentUser() method
instead of doing their own isAdmin checks.

The goal is to make the css replacement accessible in other contexts,
for example for the manifest.

squashed a whole bunch of PRs

closes #1907
closes #1908
closes #1909
closes #1910
closes #1911
closes #1912
closes #1913
closes #1914
closes #1915
closes #1916
closes #1917
closes #1918
closes #1919
closes #1920
closes #1921
closes #1922

ran svgo over all the admin svg files to strip all unneeded cruft.

using icons from https://materialdesignicons.com/