HTML: use nbsp instead of "" as alt text for inactive nav links

since w3m shows the filename instead of ""
update docs

array2hashset is in HashSet.pm now
pass icon filenames to MailcollectionParser to avoid their deletion
config: archive_message_title: pass message instead of identify value

to avoid over-eagerly calculating the value, and perhaps more
flexibility
cache identify value
Revert "cache identify value"

No measurable benefit.

This reverts commit 61a2a62cb87e6f270a6d95bc0bd6185a831cbc12.
HTML: use identify value instead of (message-)id for link anchors