Activate the plugins before the theme

This ensures that plugins required for a theme are activate before the theme that requires them.

Goes towards fixing habari/habari#591
Don't disable the checkbox when the plugin meets the provides requirement

Disabling the checkbox stops the data from being submitted with the POST so the plugin is never activated and the subsequent theme activation fails.

This is the second part of fixing habari/habari#591
Merge pull request #69 from habari/fix-theme-plugin

Fix installation with a theme that requires a plugin.