Ensure attachment descriptions fill full width

Was noticeable by e.g. the scrollbar being shown somewhere inside the
container and not being able to scroll near the edges if each line of
text was sufficiently short.

Directly implements the suggesstion from
#456 (comment)

src/components/attachment/attachment.scss

@@ -129,6 +129,8 @@
       max-height: 200px;
       overflow-y: auto;
       scrollbar-color: var(--border) #0000;
+      width: 100%;
+      box-sizing: border-box;
 
       .status-popover & {
         text-overflow: ellipsis;