Setup scaffolding for a media manager

[josecelano]

Parent issue: #657

Context

Some issues depend on having a way to upload files to the Index and server them via the API.

For example:

• Customisable logo #637
• Customisable favicon #668

Proposal

We need to add a media manager where the admin can upload public files.

[josecelano]

Hi @da2ce7, I forgot to ask you about this topic yesterday. This is also a "big" change. I guess for these simple cases, we could use the database but not the dinamic settings. I would use settings is the value is just an external URL.