Add a new chapter on speakers, relation and membrane

[pbauer]

Add a chapter that refactors parts of the website once it went online. The plan is to use a speaker-type as members using membrane.

Topics:

• Refactoring of code and functionality
• Relations II (automatic relations)
• Members as content

Tasks:

• Create a new content-type 'speaker' chapter
• Use the behaviors from dexterity.membrane to create members from speakers
• Enable self-registration as speakers
• Use a subscriber to automatically create a relation to the speaker on creation of a talk
• Remove speaker-fields from ct talk (the data in the old fields will still exists as attributes on the instances)
• create speaker-view
  • show their talks if they are published
  • show review_state of talks (only for spakers)
• modify talks-view and talks-list to show a link to the speaker-page

Add migration-steps

• Create speakers from the information stored on talks
• Link these speakers to their talks
• Send membership-emails to these newly-created speakers