Sidebar Container: Run the fixed sidebar check after expanding the table of contents

[ryelle]

Fixes #409 — When the page was loaded initially, the isSidebarWithinViewport check was run on the collapsed sidebar, so the height check in isSidebarWithinViewport passed and the sidebar is set as fixed. Then, a few lines later, the collapse/expand toggle logic expanded the sidebar on large screens, and now it could be too tall for the fixed sidebar behavior.

This fix just swaps the initial isSidebarWithinViewport to after the toggle button logic, so that the menu is in it's correct state when the height is measured.

To test:

• Use the documentation test site
• View a few articles with different ToC lengths, for example http://localhost:8888/article/roles-and-capabilities/, http://localhost:8888/article/before-you-create-a-network/
• Try at different screen sizes

[adamwoodnz]

Nice work, LGTM