Started to implement changes picked up from website.

The drawer tabs have also been updated to follow the spec. Almost
everything looks terrible at the moment though, due to conflicts with
existing styling.
Everything in app bar now displays correctly.
Fixed disabled game menu styling.
Filters layout tweaks, fixed plugin menu checkbox styling.

Also fixed styling of summary/general messages sidebar items, and added
missing menu classes to dropdown menus.
Improved plugin sidebar item display.

Icons now appear on the right, text overflow is hidden and ellipses
used.
Dialogs now follow Material Design spec.
A couple of em -> px spacing changes.

Plus tidying of the main CSS file into sections.
Reorganised CSS overrides.

All the overrides that increase compliance with Material Design go in
material.css, while those specific to LOOT go in overrides.css.
Moved dropdown menu overrides.

Since paper-dropdown-menu is only found in loot-dropdown-menu, the CSS
is better placed in there. Presumably there's also less searching for
matches without the /deep/ selector.
Improved toast styling, minor CSS tidy.
Improved disabled input underline colour.
Fixed message language, game type menu item styling.

The wrong item elements were being used.
CSS formatting tweak (added linebreaks)

Makes it easier to see each element's CSS.
Grey out disabled menu icons.

This is really part of the material design spec, but there's no
`disabled` attribute for paper-item as standard, so treat it as a
LOOT-specific override.
Improve styling of editable tables.
Improved card and editor styling.
Workaround for stuck ripple effect on dropdowns.

Disable the ripple.