Improve handling of long messages

[eloquence]

SecureDrop 1.3.0 implemented a maximum length on messages that can be submitted through the Source Interface. Even messages well below that 100K character limit may be too long to be managed effectively within the SecureDrop Client. We've discussed two patterns to use here:

• progressive expansion of long messages up to a certain length
• option to open messages above a certain length in disposable VM

This issue is to discuss and track implementation of these improvements.

[ninavizz]

Proposed solutions to address ux problem, available to view in this GDrive directory.

Semi-tracking w/in UX repo epic.

[zenmonkeykstop]

Still worth doing.