IndexJob model

[lukavdplas]

This is part of #1007 - building a system to handle indexing without a command line.

Beside representing the actual index (#1695), it will be useful to keep track of index-related actions. We can create an IndexJob model for this, which represents any kind of "job" related to indices. Functions:

• Keep a log of indexing tasks. Useful when you have multiple developers and multiple projects on the same server - which we do.
• Allow planning, viewing, and starting jobs through the admin site.
• Retain data so index jobs can be queued, report on their progress, etc.

The actual contents of the job can be broken up into related IndexTask objects, e.g. CreateIndexTask, PopulateIndexTask, AddAliasTask, etc. To run a job, you run each of the tasks related to it.

Goals for this issue:

• Create IndexJob models (and related task models).
• When indexing from the command line, save an IndexJob to log the action.
• Show the IndexJob as read-only in the admin site.