Move all curriculum headings up one level

One level of Markdown # heading removed across all content in
curriculum. Deep-nested layers unnecessary.
Update TOC parsing to support new content heading level
Remove header style overrides

Default back to standard sizing and font scales provided by Bootstrap
Add forced heading scale-up on slides only

Override the Bootstrap default only when headings are within a “slide”
wrapper and nowhere else.
Merge branch 'master' into heading-level-consistency

Conflicts:
	foundations/index.md
Merge pull request #225 from github/heading-level-consistency

Tidy heading level, style sizes across all curriculum