Have source as a proper field in the form

Loading it's content from the database.  So we add source as a text
field on Indicator

Also clear up the models file now we have a better understanding of the
data model:

- logframe name should be unique
- output name doesn't have to be
- delete the Source model - it has no independent existence
- delete the ForeignKey to Source from SubIndicator - in the wrong place
Fix contenteditable placeholder for newly created rows

by making the binding to events dynamic.

Also tweak the colour of the placeholder text to more closely match the
colour that chromium users.