Framework to allow EXTRA_FORM_FIELDS

[seanvoss]

Before this get's lost.

Here is a pull request based on what I think was decided in the Forum.

Register a form field within the context of the settings.py

[stephenmcd]

Awesome Sean, thanks for kicking this off. Just a few comments:

• The ID for the field type shouldn't be dynamically calculated. These get stored in the DB and so if another field type was added to mezzanine.forms, everyone's code would break, so we should make it so that the developer has to explicitly define the ID.
• The code for reading the setting and updating fields.NAMES, fields.CLASSES etc should go directly into mezzanine.forms.fields - mezzanine.boot is generally for Django-specific things that are project-wide, and have special requirements in terms of Django loading up, specifically models and admin classes. This new code is only specific to mezzanine.forms.fields, and if it lives directly in there, we're 100% guaranteed that anything else that imports from there will have the correct values.
• When we set up new settings, they should go into one of the defaults.py modules (in this case mezzanine.forms.defaults). This means that the settings automatically get documented (see http://mezzanine.jupo.org/docs/configuration.html#default-settings) and it keeps things consistant as well. It'll also mean that by using mezzanine.conf.settings.EXTRA_FORM_FIELDS, we'll always have a value for it, so we don't need to use getattr when checking that the setting is actually defined.
• Extending the last point, we probably don't need this setting commented out in the project's settings.py - the ones that are in there are generally settings people would almost always want to configure, whereas this new setting doesn't seem like it would be a commonly used one.

[seanvoss]

So, more like that then? You now have to define the ID as well.

Let me know if you want me to make the code more defensive.

[seanvoss]

Defined as.

FORMS_EXTRA_FIELDS = (
    (
        1337,
        "captcha.fields.CaptchaField",
        "captcha"
    ),
)

[stephenmcd]

Yeah that's great Sean, thanks for the update.