Fix CSS to avoid collision with nextcloud server 'date' class.

Using the '.date' collided with the nextcloud css and made it
impossible to close the OC.dialogs.filepicker.
Changed the css selection from '.date' to '.mail-message-summary .date'
to select only the needed date.

css/mail.css

@@ -326,7 +326,7 @@
 
 .mail-message-summary-from,
 .mail-message-summary-subject,
-.date {
+.mail-message-summary .date {
 	display: inline-block;
 	padding: 12px;
 	white-space: nowrap;
@@ -347,7 +347,7 @@
 	margin-left: 37px;
 }
 
-.date {
+.mail-message-summary .date {
 	position: absolute;
 	right: 0;
 	top: 0;