Add uninstall script for Linux to remove Electron user data

#### Steps to reproduce
1. Install Simplenote .deb package on Ubuntu 18.04 (the current LTS)
2. Encounter a strange bug that I'll report next with involving syncing and password changes
3. Uninstall and reinstall simple note Simplenote to see if the caches would remove and the issue would go away
4. Note that the cache was not cleared, and all account info, data, etc was retained
5. Uninstall with dpkg --purge and reinstall to ostensibly force the remove of all caches, config files etc
6. Discover that this too didn't remove all caches/config files, meaning there's something wrong in how you're building your .deb file

Start by examining the Linux .deb uninstall path and reproducing the behavior with dpkg --purge on Ubuntu 18.04. Done means uninstalling Simplenote removes the Electron caches, configuration files, account information, and other user data described in the report.

Written by the indexing model from the issue text.