Enable equation numbers

This ought to do it:

https://mathjax.readthedocs.io/en/latest/tex.html#automatic-equation-numbering

``` diff
diff --git a/relate/templates/base.html b/relate/templates/base.html
index b0f7273..e26ed4d 100644
--- a/relate/templates/base.html
+++ b/relate/templates/base.html
@@ -149,6 +149,9 @@
displayMath: [ ['$$','$$'], ["\\[","\\]"] ],
processEscapes: true
},
+ TeX: {
+ equationNumbers: "AMS"
+ }
});


```

but all I get is `[Math Processing Error]`.

cc @lukeolson

Start in relate/templates/base.html and compare the shown MathJax configuration with the automatic equation-numbering documentation linked in the issue. Reproduce the [Math Processing Error] and verify that equation numbering works without the error; no test file is mentioned.

Written by the indexing model from the issue text.