Webv4: Mark all as read

Would be nice to have a simple button press and "you sure?" confirmation for setting all echos I can see in the current view as read, so that a new scan would show nothing and all the unread message bubbles are cleared. This would even be for echos I'm not subscribed to (set to scan for new messages).

Example behavior:

• Viewing all available FTNs = clear everything
• Viewing the echos in just a single FTN = clear only those echos in this particular FTN
• Viewing the messages within a single echo = clear only those messages in the single echo

Related, if Issue #345 (closed) is ever completed, the three above would turn into six examples, one each for showing and hiding subscribed echos.