Add "Earlier files and messages deleted" notice to conversation view

[eloquence]

When the first item (message, file, reply) in the conversation view is not the first item ever created for that source, we want to show a notice near the top of the conversation view: "Earlier files and messages deleted".

Design

The proposed design for this notice can be found here. Pink text is used for annotations and not part of the design.

Acceptance criteria

• Display the notice when there are items missing at the beginning of a sequence of items in the conversation view.
• Do not display the notice for any other case.
  • Example: Given a sequence 1 2 3 4 5 or 1 2 3 4 (with strikethrough indicating deletion), the notice should not be displayed.
  • Example: Given a sequence of items 1 2 3 (i.e. all items deleted), the notice should not be displayed. (We will show a separate empty state for this case.)

User Story

As a journalist, I want to know if a message from a source follows a deleted conversation history, so that I can distinguish new from returning sources.

[sssoleileraaa]

this was resolved as part of #1263