Make html menus scroll to avoid truncation

This is for #7496 

This is an alternate solution for https://github.com/adobe/brackets/pull/7693. This isolates the change to only HTML main menus.

Using a max-height such as `calc(100vh - 34px)` (which would need to be something like `calc(~"100vh - 34px")` in LESS) was discussed, but doesn't seem to work. I don't think it really that important to have the menu go *exactly* to the bottom of the screen, anyway.

This solution does *not* resize the menu *while it is open* on window resize, but it *will* be properly resized on next open after resize is complete.

I think this is a reasonable solution to prevent HTML menu truncation for now, and we can continue to improve it as priorities allow.