[1.x] fix: validate mail settings for whitespace

[imorland]

**Fixes #4222 **

Changes proposed in this pull request:
Introduces a reusable trait ValidatesMailSettings which is used by default in SmtpDriver and MailgunDriver to ensure that leading or trailing whitespace throws a validation error, which is displayed in the mail settings UI.

I'll also create an accompanying PR to this for 2.x

For SmtpDriver, we now check for whitespace on

• mail_host
• mail_port
• mail_username *
• mail_password *

* still allows for empty string

For MailgunDriver

• mail_mailgun_secret

mail_mailgun_domain is already covered by the regex rule in place

Reviewers should focus on:

Screenshot

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.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)