Remove all unnecessary usage of BAKED_BASE_URL throughout the site.

[ikesau]

Slack discussion

In many locations, we construct an absolute URL using the BAKED_BASE_URL env variable, for (it seems) almost no point.

Marcel points out that it is needed for SVG downloads so that the links work there, and also in the admin gdocs preview in case authors want to QA some of the links in a topic page or something.

But I think we should be able to remove a lot of these instances and nothing will change except the code will get simpler and we'll have one less superstitious practice.

Impact

None on the site, except maybe a regression if it turns out it does have a purpose in one of the places we change

But also a slightly simpler codebase 🙂