Only load the WordProof admin CSS & JS on WordProof admin pages

[jdevalk]

Describe the bug
I'm seeing the WordProof admin CSS & JS on non WordProof pages:

<link rel='stylesheet' id='wordproof.admin.css-css'  href='https://joost.blog/wp-content/plugins/wordproof-timestamp/assets/css/admin.css?ver=1603938797' media='all' />

To Reproduce
Steps to reproduce the behavior:

1. Go to any other page in the admin
2. View source
3. See the styles and scripts are loaded

Expected behavior
Not loading those :)

[marijnbent]

Thanks for adding this issue Joost 🤓. Will be fixed in the next version.