New todo, fixed some Checkstyle issues (mvn checkstyle:checkstyle) and cleanup

Main:
- Added TODO: make all sort radio buttons visible again.

Constants:
- Fixed some small Checkstyle issues (after running "mvn checkstyle:checkstyle" and checking "target\checkstyle-result.xml").
- Removed SIMPLE_DATE_FORMAT_STRING since it is no longer necessary after changing the DataEntryForm class (see below).
- Removed unused constants: REPORT_ERROR_MISSING_FILES, REPORT_ERROR_MISSING_METRICS, REPORT_ERROR_MISSING_FILES_METRICS, YES_BUTTON_TEXT, NO_BUTTON_TEXT, HTML_OPENING_TAG and HTML_CLOSING_TAG.
- Removed old constants in comments: DEFAULT_REPORTS_DISPLAY_PERIOD and DEFAULT_REPORTS_DISPLAY_PERIOD_VALUE.

DataEntryForm:
- Replaced Constants.SIMPLE_DATE_FORMAT_STRING by Constants.DATE_FORMAT.toPattern() in the createDateChooser method. This means we can remove the Constants.SIMPLE_DATE_FORMAT_STRING constant (see above).

ReportUnit:
- Fixed some small Checkstyle issues.