fix: Linkify email address lists one-by-one

[larseggert]

Seems to work around a bug in mozilla/bleach.

Fixes #6307

[rjsparks]

A different solution would have been to make linkify also operate on and return lists (or make a new filter named linkify_list), but unless this starts exploding joins in many places, the return isn't that high.

[rjsparks]

Test fails are in selenium, which we'll investigate separetely.

[larseggert]

A different solution would have been to make linkify also operate on and return lists (or make a new filter named linkify_list), but unless this starts exploding joins in many places, the return isn't that high.

I tried that, but Django seemed to stringify the list when it gets passed to the filter.