Dashboards - add layout mode to dashboards

Summary:
This gets us the ability to specify a "layout mode" and which column a panel should appear in at panel add time. Changing the layout mode from a multi column view to a single column view or vice versa will reset all panels to the left most column.

You can also drag and drop where columns appear via the "arrange" mode.

We also have a new dashboard create flow. Create dashboard -> arrange mode. (As opposed to view mode.) This could all possibly use massaging.

Fixes T4996.

Test Plan:
made a dashboard with panels in multiple columns. verified correct widths for various layout modes

re-arranged collumns like whoa.

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4996

Differential Revision: https://secure.phabricator.com/D9031