Only proxy non-https images

[Linux4]

e.g imgur blocks hetzner IPs with HTTP/429, but already is https anyway

Fixes #0000

Changes proposed in this pull request:

Don't use proxying for pictures which are already served using https

Reviewers should focus on:

Screenshot

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

Required changes:

• Related Flarum core extension PR's: (Omit this section if irrelevant)