Correctly transform base64 with multiple instances of + or /

[robintown]

String.replace only replaces a single instance of the search pattern by default; we need a regex in g mode if we want to replace them all.

For https://github.com/element-hq/element-call/issues/2429

[dbkr]

🤦 Good spot

[t3chguy]

There's also replaceAll nowadays https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replaceAll

[robintown]

I saw that, it would've required me to bump our TypeScript ES target though so I decided not to bother