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).
