Front-end editing: Add plugin menu does not appear when placeholder is empty

[aaronrenner]

When trying to add a plugin to an empty placeholder using front-end editing, the "add plugin menubar" is not rendered. This is a regression since the beta-3 release.

It seems there are two lines of code in cmstags.py which prevent render_placeholder from being called. Since the "add plugin menubar" is inserted in render_placeholder, if render_placeholder isn't called the add plugin menubar won't be inserted.

My fix is to remove the two lines which are preventing render_placeholder from being called when the placeholder has no plugins.

[ojii]

this pull request does not solve the issue in an acceptable manner, there will be a fix for this shortly though. Thank you for the effort though.