Multiple newsletters: feature requests round-up

This is an effort to integrate issues #26422, #26597 and #26492 into one battle plan. I'll set these 3 issues to "duplicate", so please post any further comments in this issue.

After a lot of considerations, I think the best solution is, as Boris suggests, to provide a separate, configurable block for each newsletter. This block can show 1) a link to the term's overview page and/or 2) a link to the associated rss-feed and/or 3) the latest x issues of that newsletter.

Furthermore, instead of managing subscription to the different newsletters through the blocks, I would suggest that each block just displays a link "manage subscriptions", which leads to a central spot for managing your current subscriptions (when logged in), or for (un)subscription to various newsletters as an anonymous user. I think that if each block would show a separate subscription form, the blocks would take a lot of space and would look rather ugly. On the other hand, if there is a real need for subscription through blocks, I could make this a configurable feature.

The subscription lists to the different terms will be managable by an admin. This means that an admin can add (import), export and delete email addresses to/from the different lists. As is already implemented, an admin will also be able to inactivate an email address (global, i.e. for all newsletters) without deleting the address from the database. I think this would meet Nathan's needs.

Finally, an API will be provided to generate the subscription form, and the blocks' content. This will enable the use of custom subscription pages, information pages, blocks,... as was discussed in this issue.

Since I'll try to do a lot of this coding during my holidays, and since I'm leaving for 2 weeks to a place without access to the internet (will I survive?) this Friday (22/07), please submit any comments or thoughts before then.