Split message content into its own module

This is laying some groundwork for the rich text chatbar.

src/messagecontent/images/flight.svg

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 22 22">
+  <defs id="defs3051">
+    <style type="text/css" id="current-color-scheme">
+      .ColorScheme-Text {
+        color:#4d4d4d;
+      }
+      </style>
+  </defs>
+  <path
+     style="color:#4d4d4d;fill:currentColor;fill-opacity:1;stroke:none"
+     d="m 2.3243326,11.281529 v -0.48768 L 2.763251,10.732949 1.9829519,8.0750591 h 1.2192176 l 1.7800603,2.4506299 c 1.1867061,-0.10575 2.9423813,-0.19913 5.2670252,-0.28042 C 9.8184645,8.0019091 8.9081155,5.4090391 7.5182042,2.4666491 h 1.4996403 c 2.1134945,2.74341 3.8331395,4.98965 5.9741715,7.7786199 h 3.218737 c 0.568969,0 1.048529,0.10574 1.438679,0.317 0.2601,0.13817 0.390149,0.29667 0.390149,0.47549 0,0.17883 -0.130044,0.33732 -0.390149,0.4755 -0.39015,0.21133 -0.86971,0.31699 -1.438679,0.31699 h -3.218737 c -2.145078,2.80603 -3.840734,4.99587 -5.9741715,7.77861 H 7.5182042 c 1.3899113,-2.86108 2.3002603,-5.45396 2.7310508,-7.77861 -2.3246439,-0.0894 -4.0803191,-0.18287 -5.2670252,-0.28042 l -1.7800603,2.45061 H 1.9829519 l 0.7802991,-2.65791 z"
+     class="ColorScheme-Text"/>
+</svg>