CSS adjustments to Conpherence

Summary:
Fix Conpherence messages overlapping header in mobile mode

Before:
{F413544}
After:
{F413546}

Make Conpherence input box be one line even in desktop mode

Before:
{F413541}
After:
{F413540}

This second change, in particular, is motivated by the fact that pressing Enter sends the message, whereas the multi-line box gives the impression that the Enter key would simply introduce a line break in the message. (That's still possible via Shift+Enter, btw.)

Test Plan:

• Visit a Conpherence room with some content in mobile mode; scroll down and notice that the messages no longer slightly overlap the header.
• Visit a Conpherence room in desktop mode; verify that the input box now has the height of a single line.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Tags: #conpherence

Differential Revision: https://we.phorge.it/D25485