Images get uploaded even when no upload_files capability

[bagia]

When in the “Contributor” role, the capability upload_files is set to 0.

Problem: In the editor, we’re still able to pick images, and we get told the the upload failed. This is annoying in two ways:

• first we uploaded the image: this is time and resource consuming (I'm thinking Power and Data)
• second the user can't know why it failed

Proposed fix:

• store the upload_files capability in the data model.
• when tapping the "insert image" button, detect if the capability is denied and if it is show a tooltip with a message like "You don't have permissions to upload files on this blog"

[bummytime]

UPDATE: still an UX issue for sure. This is slightly complicated by the fact that searching the upload_files flag in blog capabilities is not always possible. e.g. Self-hosted, non-Jetpack blogs will have blog.capabilities == nil

When a contributor user attempts a media upload it will will fail correctly, however the user will see errors such as:

which still can be very confusing.