bug: Ensure CSRF tokens are URL-safe by updating base64 encoding

[Mido-sys]

Description

Including the / character in the authenticity token may trigger Cloudflare's OWASP-managed ruleset, causing otherwise valid requests to be flagged as security threats.

To Reproduce

No response

Additional Context

Details

Details

Paste the output of `buffalo info` here!

[Mido-sys]

@paganotoni, I provided a pull request for the following.

[paganotoni]

@Mido-sys I will need some time to take a look at this. Thanks for it.

[tatang26]

@Mido-sys That's a great catch. Cloudflare may block an unsafe-method request that contains an authenticity_token with slash in its value (abc/123=), thinking it's an attack on the server.
I also left a comment (just a note) in your PR about the change effect.

[Mido-sys]

@paganotoni, I have fixed the flaky failing test. It wasn't related to CSRF changes.

[paganotoni]

solved by #5