Move manuals sections into _hmrc_sections

As part of trying to separate out manuals, move the hmrc specific parts into their own file.

HMRC manuals are going to have a different design to other manuals so will use different templates. Seperating them will make the code easier to read as it won't be doing strange seeming things to account for the different data structures that HMRC's manuals and our own manuals exist in.
Remove HRMC Javascript

Simplify the JS by removing the edge case handling for HRMC manuals (which no longer need it)
Remove CSS only used for HMRC manuals

I'm trying to restructure this CSS a bit to be less complicated, as part of that I'm removing HMRC-only styles which will be sorted out when the next iteration of HMRC manuals is complete
remove div that's not required
Unify styles between index and show pages

Show and Index have a lot in common above the main body content (they both have a Summary and an optional Body which should be two-thirds width at max screen size)

This commit removes some divs that weren't necessary, and shares styles between these parts of the page so there is less duplication of css.

This commit renames 'manual-section' to 'manual-body' which seems like a more understandable name.

This commit also refactors the index page into a list of sections rather than a lot of stacked headers (which it was before because it had to accommodate different depths of content, so some would be links to other pages and some would have copy underneath them which folded out when you clicked the header - yuck.)
Remove styles only used for HMRC

The presentation of HMRC manuals is changing, so remove hmrc specific styles for now so can refactor this css into a more logical file for supporting one type of manual, to which we can later add support for a different layout

Remove headerless section handling which is overcomplicated for what we need, replace with some css to make paragraphs above h2s two-thirds width.
Remove styling from remaining hmrc manuals

The leftover markup for HMRC manuals is being picked up by the collapsible styles and javascript, so strip those classes from the markup so the hmrc manuals will be displayed painly without broken styling
Use semantic values for colours not $grey and $black

Good catch @edds, I had forgotten about these variables.
Removed duplication between hmrc templates
Merge pull request #92 from alphagov/remove-hmrc-manuals-collapsible-support

Remove hmrc manuals collapsible support