Bug 882086 - [MMS] [UX] Message thread. Spacing between the last bubble and input field should be 1.5 rem

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 875362 - [MMS] Cannot scroll to
bottom of thread list as input field grows")