Fix plugin initialization, implement single plugin mode, and support plugin ordering.

[caseycesari]

Overview

Makes a small fix to allow plugins to load, as well as implements single plugin mode and plugin ordering.

See commit messages for details.

Connects #1129

Demo

Testing Instructions

A few things to test:

• Load the site, and verify that the plugins you have install load and can be activated.
• Add the regional planning plugin. Verify that it and the identify point plugin are ordered correctly in the sidebar based on the pluginOrder setting of region.json. Reverse the order in region.json, restart the server, and verify that the plugin order is switched.
• Turn on single plugin mode in region.json and access the site. Verify that single plugin mode loads correctly.

[caseycesari]

Thanks for the review!