Add custom composer for polling feature

[bobofishbo]

Add the nodebb-plugin-composer-custom directory, which is a replication of the original built-in nodebb-plugin-composer-default, but with changes for polling and previewing polls while maintaining all previous functionalities of the default composer.

This includes writing nodebb-plugin-composer-custom/static/templates/partials/poll-modal.tpl, which generates a modal asking user for poll title, poll options. Changes in the nodebb-plugin-composer-custom/static/lib/client.js file allow for the adding and deleting of options, as well as saving the poll into the default modal. Preview and edit all work as it would in the default composer.

To test changes, one must un-link from the default composer and switch to the custom composer. Changes would include

1. src/meta/build.js: switch 2 instances of composer-default to composer-custom
2. src/install.js: switch 1 instance of composer-default to composer-custom
3. switch package.json line with composer-default to "nodebb-plugin-composer-custom": "file:nodebb-plugin-composer-custom",

For easier demonstration of the functionality, see this demo video:
https://drive.google.com/file/d/1VxbekzTjQcDLr4_fYX84Mjl-UPuUhHAf/view?usp=sharing

[bobofishbo]

demo_bo.mov