Fix timestamp's position on the chat panel with a maximized widget in IRC layout (#8464)

Fix avatar's position on the chat panel with a maximized widget in IRC layout
Fix timestamp's position on Message Edits history modal window

Also:
- Align DisambiguatedProfile with reactions row and thread summary with a variable
- Add width property as default
- Use the global variable on _IRCLayout.scss

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

res/css/views/messages/_MessageTimestamp.scss

@@ -18,4 +18,5 @@ limitations under the License.
     color: $event-timestamp-color;
     font-size: $font-10px;
     font-variant-numeric: tabular-nums;
+    width: $MessageTimestamp_width;
 }