refactor(files): share mtime for file entry components

[susnux]

Summary

• share mtime logic between both file entry components
• remove unused mtime title (done by the NcDateTime component nowadays)
• also restructure the mtime opacity calculation to work as expected also for old files. (it used to work because the default color of the column is already maxcontrast but this seem to me a bit flaky to depend of styles somewhere set in parent components).

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[susnux]

/compile