Consolidate training and support pages

[eloquence]

We currently have two separate pages without cross-links:
https://securedrop.org/training/
https://securedrop.org/priority-support/

At minimum these should be cross-linked, but it may make more sense to consolidate them under a single "Training and Support" rubric.

[nathandyer]

I definitely agree that it makes sense to consolidate these, since they are so closely related (and since training is such a large component of the priority support package).

If there are no objections, I'll go ahead and put together a draft version of a consolidated page, and then we can all review it and make sure it's the right direction before we make it live.