Add Metadata selection before file upload

[michaelsaccone]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Before an admin uploads his documents to the RabbitHole, it's cool to be able to choose some metadata or to manually change the source of the document.
Sometimes, instead of using the local filename, it's necessary to use other names such as the website where the file was downloaded or other info that, later, the agent can use to enrich the responses

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings