Use PHP timezone conversor instead of relying on RrdGraphJS

[DeadZombie14]

This fixes a frontend issue caused by RrdGraphJS: certain timezones will not work properly because this library isnt kept up to date, and as a result will display an incorrect DST time on certain timezones that have changed the way they work, like mine, America/Mexico_City, which was always displaying one hour ahead of what it really is due to an outdate DST configuration.

My solution is to rely on PHP for timezone conversion, we take the unix time and format it to a timezone then output its resulting date string to a Date javascript object.

Display time for America/Mexico_City before this patch:

After this patch:

This issue does not happen in older versions (before commit #15783), because it used to only rely on the server's PHP timezone.

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[CLAassistant]

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[murrant]

You could fix more things by updating the timezone data:
https://momentjs.com/timezone/