Enabling MySQL 4-byte support

After installing there is this warning in the admin interface:

MySQL is used as database but does not support 4-byte characters. To be able to handle 4-byte characters (like emojis) without issues in filenames or comments for example it is recommended to enable the 4-byte support in MySQL. For further details read the documentation page about this.

And also:

Some columns in the database are missing a conversion to big int. Due to the fact that changing column types on big tables could take some time they were not changed automatically. By running occ db:convert-filecache-bigint those pending changes could be applied manually. This operation needs to be made while the instance is offline. For further details read the documentation page about this.

• filecache.mtime
• filecache.storage_mtime