Bug 875362 - [MMS] Cannot scroll to bottom of thread list as input field grows r=julienw

This patch addresses an issue where the Compose field could grow to
completely cover the message thread. Now, a "sliver" of space is
reserved to maintain a visual affordance to the underlying thread.

This patch also ensures that messages are not occluded while the Thread
UI is in "Edit" mode.

Previously, the spacing after the final message "bubble" was set to an
arbitrary value of 7rem. This patch sets the spacing to the specified
amount, and ensures that the spacing remains consistent as the Compose
field grows and shrinks (resolving "Bug 882086 - [MMS] [UX] Message
thread. Spacing between the last bubble and input field should be 1.5
rem")