Fix aboutDialog.js error for Firefox nightlies

When opening the about dialog, there's a TypeError: can't access property "hidden", document.getElementById(...) is null ... aboutDialog.js:56:14 error which also prevents checking for updates (gAppUpdater is not initialized). It's looking for an element with id communityDesc, which was removed in patch

    Bug 10760: Integrate TorButton to TorBrowser core
    
    Bug 26321: New Circuit and New Identity menu items

This path is only hit with Firefox versions that match /a\d+$/, so we see this now in legacy/trac#33533 (moved) because of mozilla-central.

Maybe a fixup for the patch hiding communityDesc instead of removing would be enough for this.

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