Steps left for seemless 10.0 updater

[MorrisJobke]

• remove data/updater-INSTANCEID/.step once the upgrade is done (best via single time background job because then we know the cron.php succeeds again -> means upgrade was successful)
• revert 202ae42 (brings back the "Open updater" button)

cc @LukasReschke

[MorrisJobke]

• master: Add repair step and revert "Open updater" button #1499
• stable10: [stable10] Add repair step and revert "Open updater" button #1500
• stable9: [stable9] Add repair step and revert "Open updater" button #1501

[LukasReschke]

All merged => Close.