Adopt org-project-capture over direct use of org-projectile

[colonelpanic8]

org-projectile has been generalized into org-project-capture, which supports both projectile and project as backends.

The variables and functions defined in org-projectile other those related to the org-project-capture-projectile-backend class are now all deprecated. They should be replaced with the equivalent ones from org-project-capture.

Also, it might make sense to allow users to use org-project-capture with the project backend which may require slightly different configuration.