Add link to 2015 Fellows announcement post

**Previously in this longstanding PR:** Integrate source-code-policy as submodule

~~This depends on cfpb/source-code-policy#5 and, once that is merged, this needs additional work to change the submodule URL/commit to the canonical ones.~~

Ready to merge.

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Now includes a link to Ashwin's post on consumerfinance.gov announcing the application period for the next fellowship.

I updated the index template to allow for linking directly to external items instead of a post on CFPB Open Tech. The post page contains the full content of the cf.gov post, however, in case someone accidentally stumbles on the cfpb.github.io permalink.