Make blocks reusable + save blocks' layout as "fixed", chained or templates

These are 4 ideas in one post:

  1. add the option to save blocks in order to reuse them on different pages, for instance like this:
  2. add an option to switch the layout of a block, e.g. the ordering of pubs or the content of a banner, to “fixed”. Let them reappear in a “fixed blocks” menu when managing a page.
  3. Chain 2 or more blocks together as a form of fixed block. cf. the first banner blocks and pub block here: If I move one block, move the other bocks, too.
  4. Introduce block templates: e.g. in pub blocks order according to language or to number of comments.