discard reply box text when changed to offline mode

[deeplow]

Description

Consider edge-case when the journalist has typed stuff in the reply box and and logs out.

Following a discussion in https://github.com/freedomofpress/securedrop-client/issues/572#issuecomment-548093438 a more long term solution would be a dialog, but that is still in design. This issue is for the temporary solution of discarding the text

Steps to Reproduce

1. open client (logged in)
2. select source
3. type reply (but don't send it)
4. log out

Expected Behavior

Text is discarded and default placeholder show like so.

Actual Behavior

The text you typed show up inside the disabled reply box.

[eloquence]

(Can confirm that this is still the current behavior in latest nightlies.)

[cfm]

NB. This is what most messaging apps mean by a "draft" message. Contrast with our DraftReply model, which is for messages either still outgoing or that previously failed to send.

[eloquence]

Closing, but this is the kind of thing we'll want to consider in defining offline mode behavior explicitly (#2484) for the rewrite.