Show every Discourse post w/ excerpt

[toolness]

This fixes #301.

Screenshot:

Notable limitations:

• It looks like the cooked attribute of posts has relative links, so e.g. if someone mentions @batu, Discourse will turn this into <a href="/users/batu">@batu</a>, which will 404 on NoI. So we might need to rebase the URLs or remove them entirely or something. We now rebase the URLs.
• For that matter, I have no idea what really fancy HTML in posts might look like, e.g. oneboxes.
• Text like "Read 3 replies on Discuss" no longer makes sense because it's showing the number of replies to the whole topic, not the particular post being shown, so now we always just say "Read more on Discuss". Might want to just go ahead and drop posts_count from the model because it's not useful anymore.
• The code currently deals with the case in which post_number is null, which is useful for migrating existing deployments. However, since "existing deployments" just means noi-demo.thegovlab.org and that's fully under our control, it might be easier to just simplify the code and require post_number to be non-null.

[toolness]

Given that this feature is still evolving, I'm going to merge this PR now and we'll see how well it works in practice.