msglist: Set message-sender-name color to match web

I overlooked this in zulip#686 because this text's color wasn't set at
this site explicitly, and my process for that PR was to search
case-insensitively for the term [color]. Instead of being set
locally, this text's color was being set by the DefaultTextStyle
that wraps the whole message list for the sake of coloring message
content.

That color:
  const HSLColor.fromAHSL(1, 0, 0, 0.15).toColor()
doesn't match web, it turns out. (Or the old Figma, which uses
true black.) Web colors this with var(--color-text-sender-name), but
that variable is undefined for light theme, so it falls back to
--color-text-default which is this color.

At this commit, if I change the message list's DefaultTextStyle to
set a conspicuous color, like orange, I only see a response from
message content, and not from sender names or anything else. That's
good, but I wonder if there's a less error-prone solution for
coloring just the message content, without regressing on
performance. I'll explore that later in this series.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -915,10 +915,11 @@ class MessageWithPossibleSender extends StatelessWidget {
                   const SizedBox(width: 8),
                   Flexible(
                     child: Text(message.senderFullName, // TODO get from user data
-                      style: const TextStyle(
+                      style: TextStyle(
                         fontFamily: 'Source Sans 3',
                         fontSize: 18,
                         height: (22 / 18),
+                        color: const HSLColor.fromAHSL(1, 0, 0, 0.2).toColor(),
                       ).merge(weightVariableTextStyle(context, wght: 600)),
                       overflow: TextOverflow.ellipsis)),
                   if (sender?.isBot ?? false) ...[