Remove migrate_to_db at upgrade

[yalh76]

Executing this at every upgrade erase all custom config of Pleroma made with Admin FE

ynh_exec_warn_less ynh_exec_as $app -s $SHELL -lc "$final_path/live/bin/pleroma_ctl config migrate_to_db"

Problem

• All customization of Instance made with Admin FE are lost (reverted to default): alternative frontend, attachement max size, custom logo, etc.
• Close #243

Solution

• Exec migrate_to_db only at install and not at upgrade. Note: this partialy revert #224

PR Status

• [ ] Code finished and ready to be reviewed/tested
• [ ] The fix/enhancement were manually tested (if applicable)

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[yalh76]

!testme

[yunohost-bot]

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