Add delete button to versions

@seldaek indicated on IRC that he would very much love to be able to delete
versions using the web interface of packagist. Since I figured it shouldn't
be much work and he just deleted a version for me I decided to throw some time
against the issue and behold! A new button!

The button only appears if the current user is maintainer or has the
ROLE_EDIT_PACKAGES role; the action executing this code is also protected using
same credentials.

This button replicates the behaviour of the clearVersionsCommand as indicated
by Seldaek.

I have tested this in a local setup.
Fix comments from code review
Refactor button visibility business logic to template

During a code review by @Stof he indicated that it is desirable to follow suit
with the rest of the application and have the business logic determining
whether the 'delete' button is visible in the template instead of the Controller.
Handle case when there are no popular packages

Change-Id: I6957791ed9f4d66a2fffe5e9b64bbc5a4be6710e
Update vendors
Add link to existing package in error message, fixes #279
Merge remote-tracking branch 'denisura/master'
Fix CS
Merge remote-tracking branch 'mvriel/add_delete_button'

Conflicts:
	src/Packagist/WebBundle/Resources/views/Web/versionDetails.html.twig
Add CSRF protection, restyle delete version button
Merge remote-tracking branch 'origin/master'