Update ``Application`` section documentation.

Document the expected behaviour on membershiop.
Improve wording on ``Application`` edit form.

Before the purpose of the form was confusing.
Add ``MembershipForm`` to validate collaborators for an app.

https://github.com/madewithbytes/us_ignite/issues/74

The form receives a list of email addresses separated
by a new line.

The email address is validated with the registered users,
since being a member of an application gives the account
extra permissions for the application.

The ``clean_collaborators`` methods transform the payload
in a list of registered users. If an invalid email address
or a non-registered user is found, the form returns an
error.
Add ``display_name`` to the ``Profile``.

The display name will show a sanitized version
of the user details when available.

The user display name when available, otherwise
a sanitised string.
Add ``display_name`` html include.

The include validates and shows a link to the user
profile when availabls.
Implement add application membership feature.

https://github.com/madewithbytes/us_ignite/issues/74

The registerd users can be added as collaborators in
an ``Application`` via this view.

The collaborators can only be added by the ``owner``
of the app.

Will fail with invalid addresses or non-registered
members.
List collaborators and metadata in app profile.
Add membership fixtures to test helper.
Add remove application membership mechanics.

https://github.com/madewithbytes/us_ignite/issues/74

The members are listed in the membership management
page. They can be removed via a ``POST`` request.

Closes #74.
Add link to remove user account in edit profile section.

The link appears at the bottom. It will show a
confirmation page before removal.