Create Studio External Templates

As a user, I should be able to share an external template. 

An external template can be created if the Nuxeo User has the suitable permission. An external template can be created only on Nuxeo Studio Releases (Same as before)

New properties and updates on existing properties:

• Description: Markdown syntax should be available.
• Features: It should be a text area field where user can indicates new features on an external template. Markdown syntax should be available.
• Categories: Should list the categories used in the Nuxeo Community Cookbook (https://github.com/nuxeo/nuxeo-studio-community-cookbook#module-listing)
• Icon: Blob property. We should ideally filter on file extension to allow only picture file extension
• Screenshots: Multivalued Blob property. We should ideally filter on file extension to allow only picture file extension
• Documentation: URL link to get more details on a template
• Vendor: No longer a free text input: it should be a suggestion which list all the organisation of the current user
• Selected version: Display Nuxeo server distribution label and package label instead of IDs
• Visibility: For this first version, the visibility can only be public, but we need to anticipate that this field will have other possible values, such as "Restricted" to allow sharing only for specific organizations