Refactor & make base64 functions browser-safe

We had two identical sets of base64 functions in the js-sdk, both
using Buffer which isn't really available in the browser unless you're
using an old webpack (ie. what element-web uses). This PR:

 * Takes the crypto base64 file and moves it out of crypto (because
   we use base64 for much more than just crypto)
 * Makes them work in a browser without the Buffer global
 * Removes the other base64 functions
 * Changes everything to use the new common ones
 * Adds a comment explaining why the function is kinda ugly and how
   soul destroyingly awful the JS ecosystem is.
 * Runs the tests with both impls
 * Changes the test to not just test the decoder against the encoder
 * Adds explicit support & tests for (decoding) base64Url (I'll add an
   encode method later, no need for that to go in this PR too).

src/client.ts

@@ -47,7 +47,7 @@ import { Direction, EventTimeline } from "./models/event-timeline";
 import { IActionsObject, PushProcessor } from "./pushprocessor";
 import { AutoDiscovery, AutoDiscoveryAction } from "./autodiscovery";
 import * as olmlib from "./crypto/olmlib";
-import { decodeBase64, encodeBase64 } from "./crypto/olmlib";
+import { decodeBase64, encodeBase64 } from "./base64";
 import { IExportedDevice as IExportedOlmDevice } from "./crypto/OlmDevice";
 import { IOlmDevice } from "./crypto/algorithms/megolm";
 import { TypedReEmitter } from "./ReEmitter";