Major enhancements to Maniphest

See the Maniphest task at https://secure.phabricator.com/T2575.

Added PageUp-specific modifications.

Added change control board export.

Added support for date, person and description fields.

Added handling array storage (for person type) in aux fields.

Fixed detail view and task history for custom field types.

Added check not to omit user in Git URI if user is "git".

This is mainly for usability with Gitolite installations, where the
user is the same for everyone, and is differentiated by the
private key that is used.

Added field grouping support and fixed person fields.

Added support for read-only fields.

Update change control board infrastructure for generic use.

Generalized default CCB download filename.

Finished implementation on filtering the PageUp Status field in Maniphest.

Fixed issue with custom queries causing page crash.

Implemented custom attributes for Maniphest custom query page.

Fixed bug with date field initial value being forced where it
shouldn't be.

Generalised the task list modifications w/ custom columns.