Description of problem:
Whenever user is about to change context, and has unsaved change in editor, Zanata should prompt the user for actions.

Version-Release number of selected component (if applicable):
2.0-

How reproducible:
Always

Steps to Reproduce:
1. Go to translation editor and type in some text (one row become unsaved)
2. Type something in the search box and hit enter
  
Actual results:
Page is reloaded with messages matching the search string but unsaved change is lost

Expected results:
Similar to filter by state option change, user should be prompted to "save change", "save as fuzzy", "discard change" and "cancel" before reloading the page

Additional info:
There may be other occasion that current page context is about to change (current page contents is about to change). If there is unsaved changes, we should ask the user about what action he wants to take before making the context switch.