Mail: Collection of issues, ideas, glitches #1457

This issue is supposed to collect everything we notice about the current state of the mail app: issues, ideas, glitches. Just edit the list.

Ideas

• show number of messages (total / unread). mobile vs desktop? Apple Mail shows the total number of messages only on desktop, not mobile.
• show number of unread messages in mailbox tree

Bugs / Glitches

• click on account loads the inbox of the account instead of showing the mailbox tree of the account (glitch on mobile)
• click on an email with an appointment without an organizer crashes the mail navigation
• For very small mobile screens, truncate to the sender does not work and receivedAtRelativeDate is split across multiple lines.
• the AppLoadingSpinner components gets used a lot. This component however is supposed to be used once per app because it has a fixed id.
• the full 100% view of the nav doesn't work on mobile
• hovers on rows and actions are often missing