Streamline the redirect situation

We have too many different ways to set up redirects (so that legacy URLs still work but land you on a newer page):

* in the Apache config (in another repo managed by ops)
* in a handful of leftover jekyll pages (those won’t be a problem for long)
* in ~50 gatsby pages (`$ git grep 'http-equiv.*refresh' -- "./gatsby/*"`)
* and worst of all we have [a script that generates another 50](https://github.com/matrix-org/matrix.org/blob/master/scripts/generate-redirects.js).

We don’t need to settle on a single one (I reckon some belong in Apache config and some don’t), but we probably want to merge the last three methods.

Start with scripts/generate-redirects.js and run the issue's git grep command to inventory the roughly 50 Gatsby pages using http-equiv refresh. Compare those sources with the leftover Jekyll pages and determine how the last three redirect methods can be merged. Done means the redundant page and generated-script approaches are consolidated, with legacy URLs still reaching their newer destinations.

Written by the indexing model from the issue text.