Add BTSync and MB plugin, Plex plugin cleanup

**ADDED PLUGINS**
* Add BTSync PBI source to the official FreeNAS plugins repository
* Add "Media Browser" PBI source to the official FreeNAS plugins repository

*I willl continue to maintain these plugins and build updated PBIs when needed, but wanted to start merging in my code so others could build updates when needed.*

**CHANGES (Plex)**
* Remove no-longer needed and never needed code from install scripts
* Change datadirectory from /usr/pbi/plexmediaserver-amd64/plexdata to /var/db/plexdata.
    * This would make Plex plugin updates from FreeNAS much faster. The datadirectory (which is several GBs for most) would no longer be compressed, backed-up, and uncompressed which all happened during updates and made updates to the Plex plugin extremely slow. Instead, user data stays safely in /var/db/plexdata while plugin code stays in /usr/pbi/plexmediaserver-amd64.

**FUTURE ISSUE MITIGATION (Plex)**
* **PROBLEM**
    * If current Plex plugin users update to future versions of the Plex plugin incorporating this change, their library will look as if it disappeared and updates would still be slow because of old data in /usr/pbi/plexmediaserver-amd64/plexdata.
* **SOLUTION**
    * I recommend when the next version of Plex Server is released we *'rename'* the plugin used in the repo from "plexmediaserver" to "PlexMediaServer" which will both make it more readable (*see my other plugins in the Repo*) and cause current Plex users to not be presented with the update button.