Improve plugin logic/clean up repository

Hi @stevejonesdev,

There are a few things that could (and should) be improved in this plugin:

1. The main plugin file should be called enable-contributor-uploads.php instead of functions.php. See https://codex.ww.wp.xz.cn/Writing_a_Plugin#Plugin_Files for best practices.
2. Currently, \WP_Role->add_cap(); is executed on every request (using the admin_init hook). From the add_cap code reference:
   

   NB: This setting is saved to the database, so it might be better to run this on theme/plugin activation

   Simply use an activation hook.

3. Implement uninstall routine. As I said, \WP_Role->add_cap(); persists changes to the database, so if the user deactivates or uninstalls the plugin, the capability should be removed. Use \WP_Role->remove_cap in a register_uninstall_hook callback. Also see Uninstall Methods.
4. Remove assets/ folder from the plugin’s source code. There’s a dedicated root folder of the same name to store screenshots. From How Your Plugin Assets Work:
   

   All files should be placed into the assets directory of your SVN directory and will work for all versions of your plugin. This is a top level directory, just like trunk. You would not place the screenshots into trunk/assets or tags/1.0/assets.

5. Remove the empty rwc-contributor-uploads folder.
6. Somehow the trunk/ folder was copied to tags/1.1. Remove it.

Please consider fixing/improving this. Thanks!