Add a rake task that prioritizes solr reindexing to minimize downtime

[fbacall]

Summary of changes

• Adds tess:reindex which indexes upcoming events and materials before other resources.

Motivation and context

When running sunspot:reindex it will drop the index of each model and then reindex every resource, which can lead to long periods of time where the index pages are blank while it indexes e.g. thousands of expired events.

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.