pithos: Change default disposition type to inline

Since version 0.15 a specific content disposition type can be enforced in
requests for an object's content, using the 'disposition-type' optional
parameter.

If no disposition-type is specifically requested or an invalid value is passed,
the default disposition type used to be 'attachment'.

This commit changes the default disposition type to 'inline'.
docs: Update Changelog
astakos: Create system project on user acceptance

Delay the creation of user's system project until the user is accepted,
to avoid polluting the database with unneeded project objects.

When creating the system project, extra checks are needed for possible
UUID collisions. If an existing project is found with the same UUID as
the user-to-be-accepted, the user's UUID is changed on the fly. However,
the probability of a UUID collision is close to zero.
astakos: Change the display name of system projects

Change the name of projects from "base:UUID" to "system:UUID", so as
to avoid confusion, since the name is visible in management commands.