Display issues with the Portal Builder menu on small screens

[robyngit]

Describe the bug & screenshots
The mobile menu for the Portal Builder no longer looks or works as it was designed. There are several issue that apply to all the themes:

• Clicking on the "Pages" part of the menu does not expand or collapse the list of portal pages.
  This means that if the menu happens to be expanded, and the user scrolls up, the entire menu is visible and it blocks most of the screen. If the menu is not expanded, the user can't access any of the portal pages except the one they are on.

• There is a margin on the left of the menu

• The menu sometimes disappears completely if you switch from a narrow to wide screen.

To reproduce this:

1. Resize your browser window to make it narrow enough for the mobile menu to appear
2. Scroll down so the mobile menu becomes hidden
3. Resize your browser to make it wide enough for the mobile menu to disappear. Now there is no visible menu.

• The behaviour where the menu is toggled on scroll is a little too responsive

Less important, but I think that the menu is hidden/displayed too quickly when the user scrolls. Any amount of scrolling toggles the menu and I find it a little distracting. It might be better to have a higher threshold before toggling the menu.