Use base64url encoding for encrypted username

[brendanny]

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• My contribution is not related to translations.
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Description

Changes the base64 encoding used for encrypted username links (https://signal.me/#eu/<encrypted username>) from base64 to base64url encoding to match Android and iOS behavior. (https://community.signalusers.org/t/beta-feedback-for-the-upcoming-desktop-7-3-release/59611/4?u=bjoel)

[brendanny]

I didn't change any fromBase64() calls because Buffer can correctly handle base64url (RFC 4648 section 5) and base64 (RFC 4648 section 4) encoded strings. [1]

I didn't use toWebSafeBase64() [2] because Buffer already has support for base64url. If needed, I can open a separate PR to switch the group link generator to mirror this PR's behavior.

[1] https://nodejs.org/api/buffer.html#buffers-and-character-encodings
[2] https://github.com/signalapp/Signal-Desktop/blob/main/ts/util/webSafeBase64.ts

[brendanny]

Also, separate issue, but as I noted in the beta feedback thread, yarn test on macOS fails timestamp tests if your locale date/time settings aren't default

[indutny-signal]

Thank you so much for this contribution! We merged this internally and it will be released to beta in about one week!