Prevent column mapping dialog from exceeding window

[SoryRawyer]

Fixes #7521

Changes proposed in this pull request:

• the column mapping dialog no longer exceeds the height of the window
• the contents of the dialog will scroll instead of going off the page

Before:

After:

[tfmorris]

I need to build this to test locally, but just a quick question based on the screenshots - where is the scroll bar?

[SoryRawyer]

The other thing I noticed is that Created columns section at the bottom is not aligned the same way as the rest of the columns. I know that's a different bug, but if it's easy to incorporate the fix here, let's do it. Otherwise, we can create a new issue for it.

I think this should be its own issue. My own CSS skills are pretty rusty, but I think it's worth looking into some of the class styles we reuse in the column mapping dialog (i.e. grid-layout and layout-normal) and try to update those.