This solves some TODOs by moving shared code out into
`_blackbox_common.sh`.
New VCS commands were added, `vcs_ignore` and `vcs_notice` (the
opposite of ignore).
Made some utility functions
* `remove_filename_from_cryptlist` - The opposite of `add_file_to_cryptlist`
* `remove_line` - Removes a single line from a text file
