Theme improvements

• Removes unused #ephy-page-menu-button and button.active-menu styling. They are not mentioned anywhere in epiphany code outside of css/scss, probably some leftover, like the old pre-popover menu button.

• Splits out Adwaita-specific theming of bookmark tags into Adwaita.scss, and implements a simple public color-based style in shared.scss as fallback. The goal is to stop using Adwaita colors in shared.scss.

• Replaces a few remaining Adwaita colors in shared.scss with the matching public colors.

NOTE: these public color changes are necessary, because CSS files have been edited manually before, and so we need to port the same changes to SASS, so that it still generates the same thing.

• Fixes inconsistent indents, comments etc.
• Fixes reader mode and bookmark icon colors in location bar. It's based on Adwaita entry icons (like the lock) style, but is not using any Adwaita colors, so it should match perfectly on Adwaita (both light and dark) and look good enough on other themes.
• Ports parse-sass.sh script to sassc, like in gtk and gnome-shell.
• Regenerates CSS from the changed SASS using the changed script.
• Changes reader mode icon to be opaque (patch by @jimmac, unchanged)

shared.scss file is now very simple and can be ported to pure CSS easily. Adwaita.scss, not so much, because it inherits from Adwaita styles.

Screenshots: Bookmark tags are exactly same as before on Adwaita, but are a little bit different on other themes.