Standardise imports and move partials

Move partials into a helper folder so that it is obvious as to which
files are actually compiled.

Standardise the naming of partial files to match the Sass convention of
starting with an underscore.

Update `import` lines to call partials without a leading underscore and
without a file extension.
Call header and footer directly

Removes extra extraction which just causes you to have to open more
files to find the actuall styles.
Update print Sass files

- Move shared code into the shared helper
- Use standard import lines to match the other Sass files
Move header and footer Sass files

To match the calls to the files move them into the helpers folder.
Unify calls to mixin files

The mixin files only need to be called in the main compiled files as the
partial files will always be included by one of them. This saves
including the same file multiple times.
Update print Sass file to work

This needs the govuk_frontend_toolkit typography not the local one.
Merge pull request #414 from alphagov/tidy-sass

Tidy up Sass