Donation pledge groups -- proposal

[Davi Leal]

* Added "Donation pledge group" creation and update support.
* Volunteers entries can not be created yet.
* Job Offers works as usual.


The new code is part of the "phase 1", that is to say, without bank support.


I have created a demo entry at [1] so that you can play with it:

  [1] http://gnuherds.org/pledges?id=48

Created from "Click to create new pledge entry" at [2]:

  [2] http://gnuherds.org/pledges


The goal of this email is to get feedback to modify or fix anything before 
committing it and check we are not doing something that has not sense.



Unit tests:
  * Click "donation pledge" at [1].
  * Click "I do it" at [1].
  * Create a new entry at [2].


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