Add guideline regarding no branding

[coffee2code]

As originally proposed by @gagan0123, the Photo Directory has had a policy of rejecting photos prominently featuring branding or branded products. However, the policy is not explicitly stated anywhere.

Adding this new guideline requires these changes:

1. The guidelines page should list the new guideline.
2. A new checkbox should be added to the photo submission form to confirm that the photo contributor is aware of, and abides by, the guideline.
3. Moderators should have a rejection reason that corresponds specifically to the guideline. That reason should have an explanation provided in the rejection email.

[coffee2code]

In 12808:

Photo Directory, Rejection: Add rejection reason for prominent branding.

Props gagan0123, coffee2code.
See #7176.

[coffee2code]

In 12809:

Photo Directory, Uploads: Add submission checkbox for branding guideline.

Props gagan0123, coffee2code.
Fixes #7176.