[breaking] trust setting to indicate whether input text is trusted (#1794)

* trust option to indicate whether input text is trusted

* Revamp into trust contexts beyond just command

* Document new trust function style

* Fix screenshot testing

* Use trust setting in \url and \href

* Check `isTrusted` in `\url` and `\href` (so now disabled by default)
* Automatically compute `protocol` from `url` in `isTrusted`, so it
  doesn't need to be passed into every context.

* Document untrusted features in support list/table

* Existing tests trust by default

* remove allowedProtocols and fix flow errors

* remove 'allowedProtocols' from documentation

* add a comment about a flow error, rename urlToProtocol to protocolFromUrl

* add tests test that use function version of trust option

* default trust to false in MathML tests

* fix test title, remove 'trust: false' from test settings since it's the default

test/screenshotter/test.html

@@ -70,7 +70,8 @@
 
         var settings = {
             displayMode: !!query["display"],
-            throwOnError: !query["noThrow"]
+            throwOnError: !query["noThrow"],
+            trust: true  // trust test inputs
         };
         if (query["errorColor"]) {
             settings.errorColor = query["errorColor"];