mrhlpr.frontend: Fix string representation of ActionType

Previously, the string representation for ActionType would be something like ActionType.OK instead of [OK ] as intended. This resulted in commands like $ mrhlpr status getting an output like this:

ActionType.OK Changes allowed
ActionType.OK Clean worktree
ActionType.NOK Rebase on master
ActionType.NOK MR-ID in commit msgs
ActionType.NOK Commit subjects follow format
   668526 doesn't match any regex
ActionType.NOK Commits are signed

instead of the intended

[OK ] Changes allowed
[OK ] Clean worktree
[NOK] Rebase on master
[NOK] MR-ID in commit msgs
[NOK] Commit subjects follow format
   668526 doesn't match any regex
[NOK] Commits are signed

Fixes 8647dd3b