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/rust-crypto/rust-crypto.ts

@@ -70,7 +70,7 @@ import { TypedReEmitter } from "../ReEmitter";
 import { randomString } from "../randomstring";
 import { ClientStoppedError } from "../errors";
 import { ISignatures } from "../@types/signed";
-import { encodeBase64 } from "../common-crypto/base64";
+import { encodeBase64 } from "../base64";
 import { DecryptionError } from "../crypto/algorithms";
 
 const ALL_VERIFICATION_METHODS = ["m.sas.v1", "m.qr_code.scan.v1", "m.qr_code.show.v1", "m.reciprocate.v1"];