Partials improvements

[gazpachoking]

This makes several changes regarding handlebars partial rendering:

• Rendering a partial page which contains metadata is now possible
• Rendering a partial page which contains other imports is now possible
• Metadata from all pages in import chain are combined before rendering
• Metadata from outer scopes overrides inner scopes (and earlier in import list overrides later)
• Preview works before saving when adding new imports
• When trying to include a wiki page that doesn't exist, an error will be rendered in the markdown
• When viewing historical revisions of a page that uses partials, the historical partials are also used.

TODO:

• [x] When editing a page to add a new import, it doesn't autorefresh. (editing again after ajax call to get new partial has completed will cause the preview to update)
• [x] Order of imports needs to be respected when combining metadata from all pages in current scope (done on python side, needs to be updated to use ordering on js side)

EDIT: Accidentally started this branch on top of markdown-it branch. That change is unrelated. rebased it.

[gazpachoking]

Fixed a bug viewing historical versions of pages, and historical page views now also use the proper historical versions of the imports they contain.

[gazpachoking]

@scragg0x Any concerns with this one? http://flexget.com is currently running this branch and it seems to be working well.