Change the default upload agreement text

[josecelano]

From: torrust/torrust-index-gui#623 (comment)

From:

"I confirm that the content I am uploading is authorized, and I have read and agree to the terms."

To:

"I comprehend and agree with this index's usage and content policy and that this upload is in full compliance and that this upload is a meaningful contribution according to the spirit and goals of this index."

As @da2ce7 suggested here.

[josecelano]

@cgbosse proposed:

"I have read and agree with the Usage Policies and Content Restrictions."