limit link schemes in in messages in e-mail push notifications

[RamReso]

Resolves #2860. This pull request limits the allowed link schemes from messages contained in e-mail push notifications.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[MadLittleMods]

Ideally: add a test case (or otherwise) demonstrate that safe_markup makes use of the this schema allowlist.

-- @DMRobertson, matrix-org/synapse#2860 (comment)

A test would be good. Have you at least tested this manually? Please provide some instructions

[MadLittleMods]

Comment is no longer relevant since we're using a version of bleach that supports this

Suggested change

# When bleach release a version with this option, we can specify schemes

[MadLittleMods]

Do we even need to define this?

The list of ALLOWED_PROTOCOLS now defaults to http, https
and mailto

-- https://github.com/mozilla/bleach/releases/tag/v1.5

And I can see it in their code: https://github.com/mozilla/bleach/blob/913ab75992b845e2c9c060c41f24d46921db4693/bleach/sanitizer.py#L37-L38

[MadLittleMods]

To make it more clear, the minimum version of bleach needed to use the protocols argument is 1.5 (https://github.com/mozilla/bleach/releases/tag/v1.5)

We're now using bleach>=3.2.0 as our minimum so we're good on this front ✅

synapse/pyproject.toml

Lines 75 to 76 in 8eb220a

	# 3.2.0 updates collections.abc imports to avoid Python 3.10 incompatibility.
	"bleach>=3.2.0",

[MadLittleMods]

Depending on the answer to #19741 (comment) we can at-least use this PR remove this entire comment (and the commented out usage).