Migrate to git-crypt

[arcticicestudio]

In #64 (PR #69) a transparent Git encryption was introduced to GPG encrypt specific files using the smudge and clean filters.
This approach only worked reasonably well with various problems, e.g. caused files to be flagged as „dirty“ (modified) after checkout even if there have been no changes. The pure Git builtin solution with filters was chosen to prevent dependence to external tools, but the latest release versions of Git causing more and more problems due to changes in the behavior e.g. how git diff works (also causing diff-so-fancy to be broken).

This issue will undo the previous solution and, against contrary to the opinion in #64, migrate the repository to use git-crypt instead. Even if this adds additional files to the repository and adds the external dependency to the tool itself, git-crypt is a stable and production proven concept that works safely and allows to really use a transparent encryption.

After playing around and testing git-crypt locally the resulting migration steps are as follows:

• 1 Delete the custom git-diff-igloocrypt script
• 2 Remove the custom [diff "igloocrypt"] section from the gitconfig file
• 3 Remove the custom filter "igloocrypt" from the gitconfig file
• 4 Update the filter and diff custom global Git attributes handler for all *.igloocrypt files to the new git-crypt setup
• 5 Decrypt all *.igloocrypt files and backup them up outside the repository
• 6 Delete all *.igloocrypt files within the repository
• 7 Initialize git-crypt for the repository: git-crypt init (default key)
• 8 Add the main Arctic Ice Studio GPG key: git-crypt add-gpg-user -n <ID> (-n flag prevents automatic commit of generated files)
• 9 Commit the new generated .git-crypt folder
• 10 Unlock the repository: git-crypt unlock
• 11 Add all backed up *.igloocrypt files back into the repository
• 12 Ensure all *.igloocrypt are tracked to be encryped: git-crypt status
• 13 Commit all *.igloocrypt files
• 14 Validate the encryption works by locking the repository again: git-crypt lock