Enable administrators to edit all text on source interface

[redshiftzero]

Description

We have an issue (#1967) for allowing admins to create a custom line of text on the first page of their source interface. However, it would be ideal if the administrator could edit the text in the entire interface, such that custom information could be displayed, e.g.: organization-specific policies, "we respond to all SecureDrop submissions within 48 hours, check back then". First suggested in #344.

Blocked by: #1419

User Stories

As a SecureDrop administrator, I want to be able to edit the text on the source interface so I can customize SecureDrop for my use case.

[eloquence]

Removing "help wanted" label for now and added "needs discussion"; we should have a conversation what level of customizability is appropriate for the SI and how it can be best managed from a UX and security perspective (e.g., admins may try to add external links or alternative submission methods).