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.
This is pedantic but there should be a space after `'/'` like this: `{{ post.url | remove_first:'/' }}`.
Also appears on line 116