Add date missing message to InboxToadlet if needed

This adds "Date missing" (or translation) in place of the date if the
message doesn't have a date header (or has one we can't parse). This
avoids leaving the field empty, and is consistent with the handling of
messages without a subject.

Fixes bug 6173
Add date to message view

This adds the message date to the list of headers in the message view,
or a message similar to the one in the folder view if the message
doesn't have a date.

Fixes bug 5836
Rewrite welcome message slightly

A few people have emailed me asking why they have to activate their
account by replying to the welcome message. This changes the welcome
message slightly in order to avoid giving people that impression.

Fixes bug 5923
Fix NPE when displaying a message without a from header

This fixes the message view so a message without a from header doesn't
cause a NPE to be thrown.

Fixes bug 6171
Don't show deleted messages in folder view

This makes InboxToadlet skip deleted messages when displaying the
message list.

Fixes bug 6170
Add from missing message to InboxToadlet if needed

Shows "Sender missing" or equivalent instead of an empty box when a
message is missing the from header.

Fixes bug 6172
Merge remote-tracking branch 'zidel/bugfixes'
Update translations

Thanks to volunteers on Transifex.