fix(core): let extensions listen to MessageSending

[imorland]

Fixes: no issue. Found while building fof/mailtrap-mail.

Changes proposed in this pull request:
The mailer dispatches MessageSending with until(), and MutateEmail returned true, so no listener registered after it ran. That is every extension's: none could change an outgoing message, or stop one by returning false. MutateEmail now returns nothing. This has been the case since #4069.

Reviewers should focus on:

• MutateEmail::handle() changes from bool to void. The class isn't marked @public.
• fof/filter's tests already worked around this by listening to MessageSent.

Screenshot
N/A

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation. (No frontend changes.)
• Frontend changes: tests are green (run yarn test in js/). (No frontend changes.)
• Frontend changes: tests have been added, or are not appropriate here. (No frontend changes.)
• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here.
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite).
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.