h2. Session expiration not visible when editing issues

As some tickets already address redmine's behavior when you are currently editing a ticket and get (in the meantime) logged out, this addresses an aditional scenario:

h3. Problem

if your session has expired, you can still click on edit for a currently shown issue and editing is still possible.
However if you hit save you will be notified your session has expired. Selecting Back (on firefox) redirects you to the login page and your edits are lost.

h3. How to test

Log in
Select an issue (viewing mode)
open redmine in a second tab and logout on this
Return to the issue tab
Select edit (editing is possible)
Select save
--> you will be notified that your session is expired and your edits are lost

h3. Desired behavior

If your session has expired at least the "edit" function should be blocked and user should get notification on invalid session. Ideally the user would be shown (when session expires during editing) the login-dialog with the possibility to authenticate, resume and save the current edit.

h2. our environment

Environment:
Redmine version 3.3.0.stable
Ruby version 2.1.4-p265 (2014-10-27) [x86_64-linux]
Rails version 4.2.6
Environment production
Database adapter Mysql2
SCM:
Git 2.7.4
Filesystem

Redmine plugins:
clipboard_image_paste 1.10
redmine_all_files 0.0.3
redmine_edit_custom_fields 0.0.4
redmine_extended_watchers 1.0.5
redmine_issues_tree 0.0.8
redmine_lightbox2 0.3.1
redmine_my_page_queries 2.1.6
redmine_people 1.2.0
redmine_silencer 0.4.1
redmine_tweaks 0.5.6
redmine_wiki_extensions 0.7.0
redmine_wiki_lists 0.0.6
redmine_xapian 1.6.5
sidebar_hide 0.0.7