Pattern Creation: Prevent uploads to the media library

[ryelle]

Fixes #37 — There are over 100 images uploaded to the Pattern Directory, these have been verified CC0 and can be used in patterns. This PR removes the ability for users to upload to the site, but leaves the ability to open the media library to insert already-uploaded images.

This PR is laying the groundwork for #375. It disables uploads for non-admins, and sets up a button to open the media modal. In #375, this will be used to open our Openverse modal.

⚠️ This only applies to non-admins, admin & superadmin users can still upload media so that we can manage the media library, in case it's necessary.

❓ Specifically requesting reviews because I'm touching capabilities, if there's a better way to achieve this let me know.

Screenshots

When media blocks are inserted, they now just have "Pattern Media Library" as an option, for example the Gallery block:

Media & Text:

In the Media & Text block, only "Open Media Library" is available to replace the image.

The Media Library itself still has the "Upload files" tab, but if you try to upload you get a permissions error:

How to test the changes in this Pull Request:

1. Try as various user levels (still need to be a user on the site to create patterns)
2. Create a new pattern or edit an existing one
3. Add any media block: Image, Gallery, Video, etc
4. You should not be able to insert via URL
5. You should not be able to upload your own file
6. You should be able to insert existing media files into the pattern
7. Play around with the pattern & blocks more - saving, editing, etc should all still work

[shaunandrews]

Could we use the primary button styling and update the language to "Choose {mediaType}" or, if not possible (or specified), just "Media Library"?

[ryelle]

Updated to use the block label/media type for the title, and primary button style for the button.