Remove part labels in project model

[jace]

As a resolution for #163, renaming the "Objective" and "Description" columns to "Abstract" and "Outline", we introduced a "labels" JSON dictionary in #164.

This is a hack that needs to be removed. Similar to the change in thinking introduced by labels in #313, we have two distinct concerns here:

1. The platform needs a summary to show in restricted spaces such as the proposal list, and a fuller body when the proposal is viewed standalone.

2. The project editor will want customisable fields (first discussed in Customisable forms #109), but this can be satisfied by offering an template for the body, without using custom fields. The word "body" need not appear anywhere. It could simply be the dominant textarea.

To do:

1. Remove the labels JsonDict (it may be renamed to part_labels by the time this ticket is picked up) and all code that depends on it
2. Rename the objective database column to abstract (a migration)
3. Introduce proposal templates (maybe in a separate ticket) (now in Proposal templates and structure #437)

Templates could either be fields stored in the Project model that are copied to a proposal, or they could be actual proposals put into a "template" state. When a new proposal is created, the user will be given the option of copying an existing template.