Homepage: Fix start module layout bugs
Remove rule in the mobile summary view that sets display: block; on a
div that's supposed to be display: flex;. This fix a layout bug in
Use flex-grow: 1; instead of height: 100%; to make the submodule bodies
grow to fill the available space when laid out horizontally (in the
desktop view). This fixes a layout bug in Safari 10 (T232549).
The display: block; rule was the fix for a different layout bug in iOS
Safari 10 (T255668), and removing it un-fixes this bug. However,
removing the height: 100%; rule also works to fix bug, so this change
does not regress it.
Follow-up to 6b461b1c3a10.