Remove default borders and other coulouring

[lgavillet]

Removing the default styles added to the tables, to allow a finer control on table displays.

Two main changes:

1. Removed the default background color on the table heads, linked via $table-head-bg. Now, just use a colouring class on the %thead tag. Example:

%table
  %thead.grey.lighten-4
    %tr
    ...

2. Removed the default borders on every table row, linked via $table-border-color. Now just use the class bordered provided by materialize. Example:

%table.bordered
  %thead
    %tr
    ...

/cc @sled @marcopluess

[marcopluess]

thanks 👌