[References] No legend for the colors used

[pafonta]

There is no explanation in the UI for the signification of the colors - white, grey, green - of the rows.

References in:

• white have no PDF attached,
• grey have a distant PDF attached,
• green have a local PDF attached.

When a user attaches locally a PDF, the reference is considered to have a distant PDF attached. Having a distant PDF attached is the same as having a user who attached a PDF at least one time.

#42 should be fixed before.