Handle post edit timeouts in a more friendly manner

[kquick]

Current behavior:

• Enter triple backticks to begin a code section
• Hit return, forgetting to enter multi-line edit mode
• Message is posted on return, but it's blank. Oops.
• Select the previous message, edit it, enter multiline, spend >5 minutes carefully constructing the post.
• Leave multi-line edit mode and hit enter to post the message
• Get the following (in red): "Post edit is only allowed for 300 seconds. Pease ask your systems administrator for details"

This is bad, because I now just lost the multi-line message I spent >5 minutes creating, and have to repeat the process (hopefully not in an edit).

It's actually not as bad as I would think, because I can use up-arrow to retrieve the multi-line input, and then just hit return, but I think we could make this better with one or more of the following:

• detect the server error and simply re-post the edit automatically (effectively nullifies the server timeout... it's not clear to me why this would be useful).
• detect the server error and add a message that I can hit up-arrow to retrieve my rejected post and re-post it.
• add a timer indicator when editing posts to show remaining time allowed for editing

There may be other possible ways to handle this, but I believe the user experience could be improved.