Updated constants

Changed USER_PLUGIN_DOWNLOAD_DIR to USER_DOWNLOADS_DIR

Moved the API URLs to PICARD_URLS dictionary

Added a URL for the plugins repository
Addressed some issues from comments

Renamed the variable 'pluginjson' to 'plugins_available'

Removed checkboxes from the plugins download list.

Renamed the file 'plugins_dl.py' to 'plugins_download.py'