Move file description template into a separate file

Move the file description template from the constant
`MediaWikiInterface.fileDescriptionPattern` to the file
`description.txt` in the main directory of the plug-in.  This allows
better customization of the file description.

 - add `MediaWikiInterface.loadFileDescriptionTemplate` that tries to
   read the file description template from the `description.txt` file
 - call `MediaWikiInterface.loadFileDescriptionTemplate` before the
   description preview and before the export

Fix https://github.com/ireas/LrMediaWiki/issues/29