Fix held messages not showing errors in view

All the class based views that are composed of both GET and POST should be rewritten to normal methods.  
That is because most of them don't render submission errors due to redirects.  
It is possible to use classes but that would mean duplicating all the GET code in the POST

See merge request !57

Fix mass subscribe view



See merge request !59

Also improve the error shown when submitting a stale msgid (page refresh)
The error used to be "404 Not found" and now is "Message could not be found"

* Unify duplicate forms

Now pagination remembers the number of items shown when switching pages

Move metrics to list summary, fixes #66

Move all information that is displayed in metrics to the summary view of a list as discussed in #66 

See merge request !51

Fixes #58

* more_info_display.html is not used anywhere
* site_settings.html is not linked to, contains nothing useful
* list_subscriptions is not linked to, contains errors.
* lists/subscriptions.html is the accompanying template
* menu/subscriptions.html is not used anywhere
* UserSummaryView is not used anywhere

Fixes #64

Fixes #61